BAGHDAD -- The head of a key U.S.-backed Sunni group was killed Monday in a double suicide bombing that claimed at least 11 other lives and highlighted the deadly precision of attacks on Sunni leaders choosing to oppose al-Qaida in Iraq.

The main target -- a former police colonel who led resistance to al-Qaida in one of its former Baghdad strongholds -- was first embraced by a bomber posing as a friend. Seconds later, the attacker stepped back and triggered an explosion, a witness said.

A suicide car bomber then struck as rescuers tried to evacuate the wounded. At least 28 people were injured in the twin blasts -- the latest in a spate of attacks against Sunnis who have joined a U.S.-supported movement against extremists and credited with helping sharply reduce violence around Iraq.

But the mounting al-Qaida backlash has stoked worries of a wider showdown brewing as extremists try to reclaim havens and intimidate the so-called "Awakening Councils" opposing them. In an audiotape released Dec. 29, Osama bin Laden warned that Sunni Arabs who join the groups will "suffer in life and in the afterlife."

Monday's bombing occurred at the entrance of a Sunni Endowment office, a government agency that cares for Sunni mosques and shrines, and near an Awakening Council office in Baghdad's northern Azamiyah district, which had been a stronghold of insurgents and a safe haven for al-Qaida in Iraq.

Sunni Endowment leader Ahmed Abdul Ghafur al-Samarrai -- who is from the same tribe as the colonel -- blamed bin Laden for encouraging the attack.

But he said the bloodshed Monday had "increased Iraqis' strength ... against those who want to create sectarian divisions."

Casualty figures from the attack differed.

Cmdr. Scott Rye, a U.S. military spokesman, said 12 people were killed and 28 were wounded. Earlier, al-Moussawi said six people were killed and 26 wounded. A police officer had put the death toll as high as 14.

The switch of allegiance by insurgents in Azamiyah was one of the most significant in a series of similar moves across Baghdad's Sunni neighborhoods.

Azamiyah is home to Iraq's most revered Sunni shrine, the mosque of Imam Abu Hanifa, and many in the area served as officers in Saddam Hussein's military and security agencies, giving an edge to the insurgency after the U.S.-led invasion in 2003.

The U.S. military has credited the general drop in violence around Iraq to several factors, including the new Sunni allies and cease-fire declared by radical Shiite cleric Muqtada al-Sadr for his Mahdi Army militia.

But commanders frequently say the improved security is fragile and could be reversed.

"It is tenuous at best," Maj. Gen. Rick Lynch, who leads U.S. forces south of Baghdad, said last week. "On any given day, the security situation could go backward by some catastrophic attack, or by the local population not seeing continuing forward progress."

Other attacks in the capital Monday killed at least seven people, police said.

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Associated Press writer Mazin Yahya in Baghdad contributed to this report.