By Sami Al-Jumaili

KERBALA, Iraq, March 9 (Reuters) - Iraq's ruling Shi'ite leaders on Friday urged neighbouring states and Western powers to join forces to help prevent an all-out civil war that could spread across the oil-rich region.

Speaking on the eve of an unprecedented Baghdad conference, Abdul Azziz al-Hakim, one of Iraq's most powerful leaders, said the meeting should boost the transition to elected government.

The conference, before a ministerial level meeting in April, will be an opportunity for the United States to sit with Iran and Syria, both accused by Washington of supporting insurgents and militias in Iraq.

"We call on the regional and international countries to support Iraq because we believe it will reflect positively on international and regional peace," Hakim told tens of thousands of black-clad Shi'ite pilgrims in the holy city of Kerbala.

"We want every country participating in this meeting to enhance the achievements made in Iraq in the last four years."

Persistent violence has marred efforts to establish a stable and democratic government since U.S.-led troops invaded Iraq in March 2003 and toppled Saddam Hussein.

Ten Iraqi policemen were missing on Friday after insurgents attacked a police station north of Baghdad, killing a policeman and wounding three more, a police source said.

The source said a group of insurgents stormed the police station in Hibhib, in Diyala province. The attack raised fears the missing men could have been kidnapped.

Religiously mixed Diyala has seen some of the worst violence between Shi'ites and Sunnis. A suicide bomber killed 30 people in a cafe in a town in Diyala on Wednesday.

The sectarian violence in the province has prompted the U.S. commander for northern Iraq to ask for more troops to clamp down on insurgent attacks.

"Could I use more forces in Diyala? No question about it and I'm in discussions about it," said Maj. Gen. Benjamin Mixon in a videolink from Iraq.

Despite opposition from U.S. Democrats, who want a timetable for withdrawal, President George W. Bush has ordered more than 20,000 more troops into battle, most of them to help stabilise Baghdad. There are currently some 140,000 U.S. troops in Iraq.

U.S., IRAN AND SYRIA

Shi'ite Prime Minister Nuri al-Maliki will host the Baghdad conference that will be attended by officials from Syria, Iran, Saudi Arabia, the United States and other permanent members of the U.N. Security Council -- Russia, China, Britain and France.

Iraq's Sunni Arab neighbours, led by Saudi Arabia, fear the violence could become a full-scale civil war and spread beyond Iraq's borders.

Washington accuses Syria and Iran of supporting militants in Iraq, a charge they deny. Bush said on Friday the U.S. message to them would be clear at the Baghdad conference.

"Our message to the Syrians and Iranians won't change at that meeting ... we expect you (Syria and Iran) to help this young democracy," he said.

The United States would "defend ourselves and the people in Iraq from weapons shipped in to cause harm", he added.

Sunni Arab countries have voiced concern about the rise of Iraq's Shi'ite majority and the growing influence of Shi'ite Iran in Iraq, while Turkey has said it will oppose secession in Iraq's autonomous Kurdistan for fear it will encourage separatism among its own Kurdish population.

Maliki made a rare tour of Baghdad's streets on Friday, visiting soldiers manning checkpoints set up under a three-week- old U.S.-backed security plan to rid the capital of Sunni Arab insurgents and Shi'ite militias.

"This conference is a sign towards stabilising the political situation and strengthening the political process that Iraqis built with their efforts and their blood," Maliki told reporters at a power plant in Baghdad's southern neighbourhood of Doura.

Maliki said security has been tightened in Baghdad ahead of the conference just outside the heavily fortified Green Zone.

Iraq state television Iraqiya reported on Friday the arrest on the outskirts of Baghdad of Abu Omar al-Baghdadi, leader of the self-styled Islamic State in Iraq, a body set up by al Qaeda's Iraq wing and other Sunni militant groups in October.

Braving suicide attacks that have killed more than 200 people in four days, Shi'ite pilgrims streamed on foot and on bus to Kerbala, south of Baghdad, ahead of the climax of the mourning rite of Arbain on Friday and Saturday.

Seas of pilgrims, holding the Koran and beating their chests, thronged outside the gold-domed Imam Hussein shrine in Kerbala, where Shi'ites believe the remains of Prophet Mohammed's grandson Hussein are buried.

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