A key Sunni ally of the US and Iraqi governments has been killed in a bomb attack in the Iraqi city of Ramadi.
President Bush endorsed Abu Risha on a recent visit
Abdul Sattar Abu Risha, 37, led what was known as the "Anbar Awakening", an alliance of Sunni Arab tribes that rose up against al-Qaeda in Iraq.
US President George Bush met and endorsed the sheikh last week in Iraq.
The White House, which has held up the movement in Anbar province as an example for the rest of Iraq, condemned his assassination as "an outrage".
Later, President Bush is expected to announce that the US may pull some 30,000 US troops out of Iraq by the middle of next year - a move made possible partly by the progress in pacifying Anbar.
Abu Risha's assassination will be a severe blow to the "Awakening" in Anbar, says the BBC's Hugh Sykes in Baghdad.
It may undermine the new movement against al-Qaeda in Iraq, he says, or it could strengthen resolve to resist the insurgents, who are regarded by an increasing number of people in Anbar as unwelcome invaders.
Abu Risha was killed, along with two bodyguards, by a roadside bomb planted near his home in Ramadi, the capital of Iraq's western Anbar province.
"The sheikh's car was totally destroyed by the explosion," Ramadi police officer Ahmed Mahmoud al-Alwani told Reuters.
It has been reported that he was the number one target of al-Qaeda in Iraq, that several attempts had previously been made on his life, and that his father and at least one of his brothers were killed by insurgents.
Several top sheikhs in the same movement were killed in a bomb attack in Baghdad in June.
Al-Qaeda forced back
Abu Risha was the leader of the Anbar Salvation Council, also known as the Anbar Awakening, an alliance of clans which sided with US forces and the Iraqi government in order to try to reclaim Anbar province from al-Qaeda.
"This is a spontaneous popular uprising against al-Qaeda, because, as you know, al-Qaeda killed our people," he told al-Arabiya TV this week.
Anbar province was once controlled by the insurgents, and was where they declared an "Islamic State of Iraq".
It was one of the deadliest parts of Iraq for US troops.
But the tribal alliance managed to force the insurgents back and reduce violence in the province dramatically.
'Model' for Iraq
The US held the group up as a success story and wants to use it as a template to organise Sunni tribes elsewhere.
Only on Monday, in his testimony to Congress, the top US commander in Iraq, General David Petraeus, said Anbar province showed how Iraq could throw off its violence and move forward.
"A year ago the province was assessed 'lost' politically," he said.
"Today, it is a model of what happens when local leaders and citizens decide to oppose al-Qaeda and reject its Taleban-like ideology."
He said the number of attacks had fallen from 1,350 in the month of October last year to little more than 200 last month.
Abu Risha expressed the same sentiment only last week, saying: "I wish we could do in all the provinces of Iraq what we did in Anbar, which is that the people and the government come together."