Armies and militia have a long history of exploiting natural resources like diamonds, timber and minerals to fund their activities. Major conflicts fuelled by revenues from resource extraction include Liberia, Sierra Leone, Angola, Ivory Coast, Democratic Republic of Congo and Cambodia. In these and other resource-fuelled wars, civilians have been murdered and subjected to human rights abuses.

Since the 1990s, activists have worked to exposed the link between natural resources and the violence perpetrated in the conflicts they are sold to support. They have had some success in pushing governments to tackle the problem.

The "blood diamonds" label, for example, led to growing pressure for an international mechanism to stop the sale of diamonds originating in conflict-torn countries. The result was the Kimberley Process - a certification system for the precious stones.

Another major project is the Extractive Industries Transparency Initiative, a coalition of governments, companies and civil society groups that aims to improve transparency in resource-rich countries through the verification and publication of company payments and government revenues from oil, gas and mining.

Global Witness, a London-based research and advocacy group that has played a key role in raising awareness about conflict resources, is pushing for an internationally agreed definition of the term.

It defines them as "natural resources whose systematic exploitation and trade in a context of conflict contribute to, benefit from or result in the commission of serious violations of human rights, violations of international humanitarian law or violations amounting to crimes under international law".

Most of the worst conflicts kept going by exploitation of natural resources in the past couple of decades are now over. But Global Witness warns that, without greater efforts to make international trade more ethical, new resource-fuelled wars could break out.

Warring parties can only profit from natural resources if they have access to markets where they can trade their bounty. Campaigners say trading regulations need to be tightened to close off global markets to conflict resources, while at the same time clamping down on smuggling.

Some are also in favour of legally binding rules for oil and mining companies, which routinely use security forces to protect their facilities in violent countries. Activists argue that, by doing so, corporations contribute to human rights abuses.

Groups like Global Witness believe existing codes of conduct for corporations aren't effective because they are voluntary and cannot be enforced. International mechanisms include the U.N. Global Compact, the OECD Guidelines for Multinational Enterprises and the Voluntary Principles on Security and Human Rights.

Improved transparency and accountability around the use of natural resources could mean that, rather than fuelling conflict and corruption, they make a contribution to reducing poverty.

When the trade in "blood" or "conflict diamonds" was at its height in the late 1990s, they accounted for as much as 15 percent of the international diamond trade. Campaigners brought the issue to the world's attention by exposing their role in brutal conflicts in Angola and Sierra Leone.

In Angola, the precious stones provided a major source of funding for rebel movement UNITA (the National Union for the Total Independence of Angola), which obtained an estimated minimum revenue of $3.72 billion from diamond sales between 1992 and 1998, according to a report, Rough Trade, issued in 1998 by Global Witness.

Much of the money was spent on munitions and other supplies, allowing UNITA to ignore 1992 election results as well as its obligations under a peace pact agreed in Lusaka. A U.N. resolution in June 1998 slapped sanctions on the group, including a ban on purchasing rough diamonds from the rebels.

In October 1999, following widespread reports that official government export certificates were being forged, diamond trading giant De Beers decided to stop buying Angolan stones on the open market.

In the former British colony of Sierra Leone, the 1991-2002 civil war - in which an estimated 50,000 people died and many more were mutilated - was also fuelled by trade in illicit gems, as rebels smuggled them into Liberia in exchange for arms.

The Liberian warlord Charles Taylor - later put on trial for war crimes in The Hague - also used the proceeds from diamonds and timber to sponsor Revolutionary United Front (RUF) rebels in neighbouring Sierra Leone. He sent his soldiers to fight alongside them, partly with the aim of gaining control of diamond fields near the Liberian border.

The U.N. Security Council slapped sanctions on "blood diamonds" from Sierra Leone in July 2000 and on stones from Liberia in March 2001. By that time, the need for an international crackdown on the trade had become clear.

The United Nations passed a resolution in 2000 calling for all rough diamonds to be certificated to ensure their legitimacy. This acted as a trigger for governments, non-governmental groups and the diamond industry to start working together to tackle the issue.

In 2003, the U.N.-backed Kimberley Process Certification System was launched. As of November 2008, the scheme had 49 members representing 75 countries.

Under its rules, member states can only trade rough diamonds with each other, and shipments must be exported and imported in tamper-resistant containers with a certificate attached.

The Kimberley Process is credited with a sharp drop in the trade in "blood diamonds". In 2004, the Canadian government, which was chair at the time, said 99.8 percent of the world's rough diamonds were certified to be from sources not involved in funding conflict.

The number of countries whose diamonds are classified as "blood diamonds" are now few, as peace has been restored in some of the worst conflict areas in west and central Africa.

The U.N. Security Council lifted the ban on diamonds from Sierra Leone in 2003, and the Liberian embargo was relaxed in April 2007.

Congo Republic was readmitted into the Kimberley Process in November 2007 after being expelled in 2004 for being unable to account for most of its exports.

Ivory Coast is the only country still subject to a U.N. diamond embargo.

Other countries that have had problems implementing the Kimberley Process are Ghana, Zimbabwe and Venezuela. For example, a U.N. group of experts on Ivory Coast found that poor controls had allowing significant volumes of blood diamonds to enter the legitimate trade via Ghana, where they were being certified as conflict-free.

A further challenge is the flourishing trade in illicit diamonds that may or may not be funding conflict. Global Witness says the easily smuggled gems have been shown to support terrorism and money laundering.

In an effort to crack down on smuggling, Kimberley members decided in November 2007 to strengthen controls and enforcement in major trading and manufacturing centres. Activists say this is necessary because it's easy to mix illicit and legal stones before they are cut and polished.

In Europe - a major destination for diamonds, along with North America - nearly three-quarters of diamond imports come to the Belgian port of Antwerp. Non-governmental groups say regulations on the stones handled here aren't tight enough, with huge transactions settled in cash. But diamond dealers insist their stones are clean, and say conflict diamonds are a problem of the past.

Logging and the sale of timber played a key role in funding the activities of Liberian warlord Charles Taylor, as well as supporting the Khmer Rouge in Cambodia.

Liberia's civil war, which left quarter of a million people dead, began when Charles Taylor and his National Patriotic Front invaded from neighbouring Ivory Coast in late 1989. Throughout Taylor's armed insurgency and his term as president following 1997 elections, he diverted profits from logging to buy arms and pay for military operations. Timber became particularly important to him after the U.N. diamond embargo in 2001.

According to Global Witness, Liberia's logging revenues in 2000 were at least $187 million, of which only $7 million went into government coffers. Allowing for production costs, about $100 million went missing. Liberian logging companies also played a key role in arms brokering - even acting as traffickers themselves - and their militias served as private armies.

The United Nations finally imposed timber sanctions on Liberia in July 2003, cutting off Taylor's lifeline - a move that contributed to his exile to Nigeria a month later. The ban was not lifted until 2006.

That year, notorious Dutch timber baron Guus van Kouwenhoven was found guilty of breaching the U.N. arms embargo on Liberia and sentenced to eight years in prison. Kouwenhoven ran two of Liberia's largest logging companies during Taylor's regime. He traded timber with companies in Europe and China, and facilitated arms deals.

In Cambodia, timber helped sustain the brutal Khmer Rouge and other factions during the country's civil war through the 1980s and into the 1990s. Much of the wood was traded across the border with Thailand.

In 1991, Khmer Rouge leader Pol Pot said: "Our state does not have sufficient capital either to expand its strength or enlarge the army... The resources (in our liberated and semi-liberated zones) absolutely must be utilised as assets."

After Cambodia's first elections in 1993, Bangkok pressured Phnom Penh to issue loggers with a certificate of origin - essentially enabling its enemy to raise funds to pursue military operations. But in May 1995, campaigners succeeded in persuading the Thai and Cambodian governments to close the border to logs. This cut off the flow of funds to the Khmer Rouge, and a year later many of its members defected to the government.

The phrase "conflict timber" only came into common usage after it was coined in 2001 by a U.N. panel of experts investigating the illegal exploitation of natural resources in Democratic Republic of Congo. The panel said a wide range of groups had made huge profits from timber sales, some of which were used to finance their role in the country's brutal 1998-2003 war.

The 1998-2003 conflict in Democratic Republic of Congo was dubbed "Africa's world war" because it involved seven countries. It has also been characterised as a "resource war" motivated by control over eastern Congo's rich natural deposits of gold, diamonds, copper, cobalt, timber, cassiterite (a tin ore) and coltan.

The market for coltan, a conductor used to make mobile telephones, boomed in 2000. Feverish mining of the mineral increased instability in eastern Congo, with locals and military groups rushing to turn a quick profit. This feature in the New York Times describes the impact of the alternative "coltan economy" in the Ituri rainforest at the time.

When the coltan price dropped a year or so later, producers switched to cassiterite, a tin ore often found in the same places. It is used in lead-free circuit boards for electronic equipment, and its rising value has been driven by China's phenomenal economic growth. Cassiterite miners earn $4-6 a day, well above the $1 a day most miners can expect to take home.

Congo's mining industry is unregulated and dangerous, and many miners work informally in appalling conditions. During the coltan boom of 2000, so many people abandoned farming to work in the mines that there were shortages of manioc flour, a local staple.

Historically, profits from Congo's resources have been extracted by whoever controls the soldiers at the mine gates. Analysts say this makes demilitarisation unattractive to those with bank accounts on the receiving end, including politicians in Kinshasa.

Western experts worry that Congo's lax security could be exploited by countries like Iran looking to get their hands on uranium that could be used for nuclear programmes. Congo provided the uranium for the U.S. atomic weapons dropped on Japan in 1945, and its Shinkolobwe mines are in the unstable eastern province of Katanga.

Ivory Coast is the world's largest cocoa exporting country. Both the government and rebels plundered the sector to fund a 2002-2003 civil war.

According to Global Witness, more than $118 million from the cocoa trade was ploughed into the war effort by the Ivorian government and the Forces Nouvelles rebel group, which controlled the north of the country.

The administration of President Laurent Gbagbo diverted more than $58 million from cocoa levies, and the rebels raised some $30 million a year by taxing cocoa shipments, the advocacy group said in a report.

Campaigners are calling for cocoa exporters - the largest of which are multinationals - to publish what they pay in taxes and levies to make it easier to root out corruption, and reduce the chance that cocoa will again fund conflict and human rights abuses.

Many states that depend on oil for a major share of their income are weak and, in some cases, authoritarian. Corruption and smuggling are often widespread, and profits and bribes from the oil industry sometimes subsidise violence.

It's rare that ordinary people benefit from their country's natural wealth, often staying mired in poverty as a small elite grow rich on oil profits - a predicament known as the "oil curse".

According to Mary Kaldor, co-director of the Centre for the Study of Global Governance at the London School of Economics, oil-reliant states like Russia, Iran and Saudi Arabia can survive through patronage, repression and nationalist rhetoric, as long as prices are high. But where there is excessive competition for revenues, sectarian conflicts may flare - in some cases, across borders.

Against a backdrop of growing demand for oil - and a finite global supply - old and emerging powers are jostling for control of what's left. Instability and conflict in the Middle East are often said to be linked to the region's substantial oil reserves.

Former U.S. Federal Reserve Chairman Alan Greenspan wrote in his 2007 book "The Age of Turbulence: Adventures in a New World": "I'm saddened that it is politically inconvenient to acknowledge what everyone knows: The Iraq war is largely about oil." He later told the Washington Post that, while securing global oil supplies was "not the [U.S.] administration's motive", he had advised the White House before the 2003 U.S.-led invasion that removing Saddam Hussein was "essential" in order to secure world oil supplies.

Africa is another hotspot of competition for oil exploration and production. In recent years, China has become a major investor in Africa's oil industry, as demand has soared in the fast-growing Asian economy.

According to a briefing on China, Africa and oil by the Council on Foreign Relations (CFR), an influential U.S. think tank, China has sought oil supplies from Sudan, Chad, Nigeria, Angola, Algeria, Gabon, Equatorial Guinea, and Congo Republic. China has established a significant presence in Sudan - purchasing around two-thirds of its oil exports - and is also building up its oil activities in West Africa.

Rights activists and poverty campaigners have criticised China for behaving irresponsibly in its trade relations with Africa, arguing that it invests without regard for political stability, human rights abuses or the welfare of local people.

For example, China has been accused of selling arms to the Sudanese government, which is involved in a conflict in its western Darfur region that has created a major humanitarian crisis. Nonetheless, analysts say Beijing is slowly shifting from its "non-interference" policy, and has started to put pressure on African governments behind the scene.

In Nigeria, Africa's largest oil producer, revenues from oil have done little to alleviate poverty since oil exploration began in the 1970s. CFR says the distribution of revenues has been undermined by corruption and mismanagement.

The Niger Delta, where most of the country's oil reserves are located, is a violent and militarised region. Militant groups frequently carry out abductions of oil workers, robberies and oilfield invasions to extract ransoms or other benefits for their villages from oil companies.