BAGHDAD — The assassination Thursday of the leader of the Sunni Arab revolt against al-Qaida militants dealt a setback to one of the few success stories in U.S. efforts to stabilize Iraq, but tribesmen in Anbar province vowed not to be deterred in fighting the terror movement.

American and Iraqi officials hoped the death of Abdul-Sattar Abu Risha would not stall the campaign to drive al-Qaida in Iraq from the vast province spreading west of Baghdad and reconcile Sunnis with the Shiite-led national government.

It was the biggest blow to the Anbar tribal alliance since a suicide bomber killed four anti-al-Qaida sheiks as they met in a Baghdad hotel in June. Abu Risha himself had escaped a suicide attack in February. But those attacks and others did not stop the campaign against al-Qaida.

Abu Risha, head of the Anbar Awakening Council who met with President Bush just 10 days earlier, died when a roadside bomb exploded near his home just west of Ramadi as he returned from his farm, police Col. Tareq Youssef said. Two bodyguards and the driver also were killed.

Moments later a car bomb exploded nearby but caused no casualties. An Interior Ministry spokesman, Maj. Gen. Abdul-Karim Khalaf, said the second bomb was intended as a backup in case Abu Risha escaped the first blast.

The attack occurred one year after the goateed, charismatic, chain-smoking young sheik organized 25 Sunni Arab clans into an alliance against al-Qaida in Iraq, seeking to drive the terror movement from sanctuaries where it had flourished after the U.S.-led invasion in 2003.

The assassination was widely assumed to have been carried out by al-Qaida, which already had killed four of Abu Risha’s brothers and six other relatives for working with the U.S. military.

U.S. officials credit Abu Risha and allied sheiks with a dramatic improvement in security in such Anbar flashpoints as Fallujah and Ramadi after years of American failure to subdue the extremists. U.S. officials now talk of using the Anbar model to organize tribal fighters elsewhere in Iraq.

Abu Risha’s allies as well as U.S. and Iraqi officials insisted the assassination would not deter them from fighting al-Qaida, and the tribal alliance appears to have gained enough momentum to survive the loss of a single figure, no matter how key.