Al-Qaeda's Iraqi affiliate said it had carried out a suicide bomb attack that killed a Sunni Muslim lawmaker last week as he toured a construction site in the westerly Anbar province, where Sunnis have been protesting against the government for three weeks.
Members of the Sunni minority accuse the Shi'ite-led government of marginalising them, and the wave of protests has raised fears that the OPEC country could again slide into widespread sectarian conflict.
The Islamic State of Iraq (ISI), an umbrella group for Al Qaeda-linked Sunni insurgents, said it was responsible for the assassination of Efan al-Esawi, describing him as the "dog of the Americans" and the "tail of the Shi'ites".
Esawi was one of the architects of the Sahwa tribal resistance that helped to subdue Al Qaeda-linked insurgents battling US troops in the Sunni heartland of Anbar at the height of the conflict of the last decade.
Posing as a worker, the attacker hugged Efan al-Esawi before detonating an explosive vest, killing them both on the spot.
"He insisted on his disbelief, treachery and war against Muslims ... until he died at the hands of the mujahideen in his current state, to be an example and a lesson for those after him," the US-based SITE Intelligence Group quoted ISI as saying in a statement on Sunday.
The ISI also claimed responsibility for other attacks in Anbar province and across the country, without giving details.
On Wednesday more than 35 people died in a suicide bomb attack and other bombings in northern Iraq and Baghdad.
Al Qaeda's Iraqi branch also voiced support for the Sunni protests and said it was fighting to "cut off the vein that is extending the life of the criminal Nusayri regime to kill your brothers in the Levant".
In neighbouring Syria, mainly Sunni rebels are fighting President Bashar al-Assad, most of whose ruling establishment are members of the Shi'ite-derived Alawite or Nusayri sect, adding to the strain on Iraq's own delicate sectarian and ethnic balance.
Sunni anti-government protests erupted in Iraq in late December after state officials arrested members of a Sunni finance minister's security team on terrorism charges. Authorities denied the arrests were political, but Sunni leaders saw them as a crackdown.
Since the fall of Sunni strongman Saddam Hussein after the U.S.-led invasion, many Sunnis feel they have been marginalised by the Shi'ite leadership.
Violence in Iraq is well down since the height of sectarian bloodletting in 2006-2007. But last year saw a rise in violent deaths for the first time in three years, with more than 4,400 people killed.