May 11 (Reuters) - Below are answers to some key questions about the Darfur rebel attack on Sudan's capital.

WHAT'S HAPPENING ON THE GROUND?

The sound of gunfire continues on the western edge of the suburb of Omdurman, across the River Nile from Khartoum. There have been reports that the Justice and Equality Movement rebels are sending reinforcements to back up a first assault that was repelled by government troops, but sustaining a fighting force over 600 km (400 miles) from rear bases is likely to prove difficult over time. President Omar Hassan al-Bashir's government has extended a curfew indefinitely, saying it wants to track down pockets of rebels in the city.

HOW SIGNIFICANT IS IT?

Regardless of whether the rebels succeed in overthrowing the government, the psychological importance of the attack is huge. Khartoum has been able to live isolated from violence for decades while the Arab-dominated central government's forces have battled rebels from the regions on the peripheries of the vast state. The government's failure to stop the rebels reaching the capital is a blow to both the administration and the prestige of the army. For the Justice and Equality Movement (JEM) rebels, it is a boost for their credibility at a time Darfur rebel movements are increasingly fractured.

WHAT IF THE ATTACK IS DEFEATED?

If the government forces break the back of the assault, a backlash against the JEM is almost inevitable. Darfur as a whole may suffer badly too. Officials have said the attack effectively spells an end to chances for peace talks with JEM. That could further complicate the planned deployment of a 26,000 strong United Nations-African Union peacekeeping force in the region.

Its coffers filled with oil money, Khartoum's government can afford the hardware to escalate the war. But the long term effectiveness of military means to end the conflict is doubtful.

Defeating insurgencies is no easy task in the vast land and measures to suppress them tend to further embitter populations against the attacking forces. The success of the Darfuri rebels in even reaching Khartoum could embolden other groups elsewhere.

WHAT IF THE REBELS WIN?

Central government has been dominated since independence in 1956 by rulers from a small group of Arab clans in the north. Breaking that hold could have enormous implications for the country, a mishmash of cultures thrown together in one state under British colonial rule.

On the one hand, a rebel takeover might accelerate the breakup of Sudan into constituent parts -- the largely Christian and animist south already has the chance of a vote to secede in 2011 under a 2005 peace deal to end decades of one civil war.

On the other hand, the Darfuris might just want to push for a greater sharing of power and wealth. Like most northern Sudanese, the Darfuris are generally Muslim but are not Arabs.

WHAT DOES IT MEAN FOR THE REGION?

Sudan has already cut ties with neighbouring Chad over the rebel attack. Both states accuse each other of backing rebels groups in their countries -- and outside analysts say that is exactly what is happening despite their repeated denials. Chad's President Idriss Deby was once close to Sudan, but his tribal roots in the Zaghawa tribe that straddles the border mean he is close to many Darfuris.

Deby was almost toppled in February by a rebel force that made a lightning attack across Chad from near the Sudanese border. He blamed Khartoum, which denied any involvement.

If the Khartoum government survives, it could mean more support for the Chadian rebels to make another push against Deby.

Sudan has often had difficult relations with its neighbours during the decades of wars and it is not only Chad that might be pleased to see the government fall.

ANY WIDER DIPLOMATIC IMPLICATIONS?

Sudan and the United States have been trying to patch up relations. But Washington has branded the Darfur conflict genocide and pushed strongly for a negotiated settlement so an escalation of conflict there is unlikely to smooth that process.

Sudan is close to China, a big investor in its oil industry and its main supplier of arms. China has come under pressure from the West and from campaigners to do more to help end the Darfur strife. It is unlikely to want to inflame the situation with the Beijing Olympics around the corner.

Khartoum also appeared to have won emergency military help from Egypt on Saturday. It is unclear how far that extends.

Chad's former colonial power France swung behind Deby when he was attacked earlier this year and remains his main backer.

France has a big component in the European Union's Eufor force near Chad's border with Darfur, which is meant to protect refugees and aid workers.

ANY IMPLICATIONS FOR BUSINESS?

Both Sudan and Chad have become oil producers in recent years, giving them access to new weaponry. But in both countries, the oil fields have been far from the fighting.

The rebel attack on Khartoum could jangle the nerves of investors who have seen hope in one of Africa's fastest growing economies. Sudan's gross domestic product grew over 10 percent a year in 2006 and 2007. The International Monetary Fund projects growth of almost 8 percent in 2008.

Investors, particularly from oil-rich Middle Eastern countries, have helped drive a construction boom in the capital that has brought shiny new buildings to the once sleepy city.

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