BAGHDAD, Iraq (CNN) -- Al Qaeda in Iraq claimed responsibility Friday for the assassination of a Sunni sheik who had united with U.S. forces to fight the terrorist group in Anbar province.
Iraqi officials pray over the coffin of Sheik Abdul Sattar Abu Reesha on Friday.
"Your brothers in the security ministry of the Islamic State of Iraq have assassinated the imam of the infidels and of the apostasy, Abdul Sattar Abu Reesha, one of the dogs carrying Bush's flag," a message posted on an Islamist Web site says.
CNN could not confirm the authenticity of the statement. Al Qaeda in Iraq calls itself the Islamic State of Iraq.
The Web site posting came the same day that mourners gathered for Abu Reesha's funeral, some chanting "We will take our revenge" and "We will continue the march of Abu Reesha." Watch the funeral »
Hundreds also walked along the highway near his home as scores of Iraqi security forces and U.S. troops kept watch.
Mouwaffaq al-Rubaie, Iraq's national security adviser, called the killing of Abu Reesha "a national disaster and a great loss for the Iraqi people," officials said.
The sheik, 39, was assassinated by a roadside bomb Thursday. At least two of his bodyguards were killed and five other escorts were wounded in the attack, which prompted a declaration of a state of emergency in Anbar province.
The Sunni leader was head of the Anbar Salvation Council -- also known as the Anbar Awakening -- a coalition of tribes that has been working with the U.S. military to counter al Qaeda in Iraq in the Sunni-dominated province.
The council, funded and supported by Iraqi Prime Minister Nuri al-Maliki, was formed last year.
On Thursday, al-Maliki blamed al Qaeda in Iraq for the killing, but said it would backfire against the insurgent group by further isolating it across the country.
"We are certain that this will only strengthen the resolve of al-Anbar's honorable sons to fight the terrorists and cleanse Iraq of their evil," the prime minister said in a written statement.
The U.S. military and the U.S. Embassy in Iraq also condemned the attack.
President Bush mentioned Abu Reesha's killing in his speech Thursday night announcing a plan to bring troop levels in Iraq back to "pre-surge" levels by next summer.
"In response, a fellow Sunni leader declared: 'We are determined to strike back and continue our work,' " he said. "And as they do, they can count on the continued support of the United States."
Meanwhile, in Washington, Pentagon planners began meetings Friday to determine which units will be leaving Iraq in the next 10 months.
The number of U.S. troops that President Bush has announced plans for withdrawal from Iraq totals about 21,500.
That's about the number of combat troops sent to Iraq as part of what the Bush administration calls the "surge" -- not including the approximately 8,000 combat support troops who also were sent as part of the 30,000-troop surge. It is unclear what will happen with them.
In Iraq on Friday, four U.S. soldiers were killed in Diyala province when an explosion occurred near their vehicle, the Multi-National Division-North said.
The number of U.S. military deaths is the Iraq war stands at 3,780, including seven Defense Department civilians. There have been 38 deaths this month. E-mail to a friend
CNN's Jamie McIntyre and Barbara Starr contributed to this report.