By Kareem Raheem

BAGHDAD (Reuters) - A prominent Iraqi Sunni cleric who urged Islamist armed groups to end their insurgency was critically wounded, and four of his bodyguards killed, when a car bomb hit his convoy in Baghdad on Sunday.

Mehdi al-Sumaidai, who once fought U.S. troops in Iraq and was part of a Sunni body issuing religious edicts, had been negotiating with insurgent groups who vowed to fight on after the withdrawal of American troops in December.

The attack came as al Qaeda's local wing, Islamic State of Iraq, has warned of a new campaign and security analysts say fighters in Iraq are benefiting from funds and morale from Islamists slipping into Syria to join the revolt against President Bashar al-Assad next door.

Al Qaeda's wing andg six Sunni insurgent groups are still fighting in Iraq, mostly against Shi'ites and security forces to try to stir up sectarian tensions, but they also target Sunnis they brand traitors allied with the Shi'ite-led government.

Police said a parked car bomb blew up when Sumaidai's convoy arrived near his residence in Yarmouk District, central western Baghdad, after he finished prayers to mark the Eid al-Fitr festival to end the Muslim holy month of Ramadan.

"Sumaidai was brought to us with four other people, all of them seriously injured. Sumaidai was wounded in many parts of his body," said a policeman at the Baghdad hospital where the cleric was taken after the blast.

Sumaidai was previously a jihadist who led a militant group that fought American troops in western Baghdad after the 2003 invasion to oust Saddam Hussein.

He spent four years in U.S. jails in Iraq before being released in 2008. During his time in jail he built good relations with fellow detainees who had led rival Shi'ite militias.

Al Qaeda in Iraq and other Sunni Islamists are hostile to Iraq's Shi'ite-led government, which many minority Sunnis feel has sidelined them from power-sharing agreements since the fall of Sunni dictator Saddam.

Iraq's al Qaeda affiliate has claimed responsibility for a wave of attacks across the country in June and July it said were part of a new offensive.

The level of violence has fallen sharply since the height of the war in 2006-2007 when sectarian slaughter killed tens of thousands of Iraqis. But insurgents have carried out at least one major, complex bombing a month since December.

(Writing by Patrick Markey in Baghdad; Editing by Alison Williams)