A double homicide bombing killed at least 12 people — including one of the most important leaders of a U.S.-backed armed group in the capital — and wounded 28 Monday outside the Baghdad office of a government agency that cares for Sunni mosques and shrines, the U.S. military said. A police officer put the death toll as high as 14.

The attack comes a little more than a week after an audiotape of Usama bin Laden was released in which he called for renewed attacks on the mostly Sunni armed groups. The U.S. military in a statement blamed the double bombing on Al Qaeda in Iraq.

Three other bombs killed five more people in the capital, police said.

In the initial attack, a suicide bomber blew himself up at the entrance to the Sunni Endowment office in Baghdad's northern Azamiyah district, said Brig. Qassim al-Moussawi. As people rushed to evacuate the wounded, a homicide car bomber detonated his explosives just yards away, he said.

The dead included Riyadh al-Samarrai, head of a local U.S.-backed armed group, said one witness, who is an employee of the Sunni Endowment. His account was corroborated by a member of the armed group, who gave his name only as Abu Omar, and by an Iraqi army official.

One of al-Samarrai's guards who witnessed the attack said the suicide bomber walked up to al-Samarrai — a former police colonel — and embraced him before detonating his explosives.

The U.S.-backed groups — predominantly Sunni Arab fighters who turned against Al Qaeda and are known as "awakening councils" — have been credited with helping reduce violence across Iraq by 60 percent since June. But they are increasingly becoming targets, with several recent bombings striking their offices and checkpoints. Monday's blast occurred near one of their offices.

The Azamiyah area had been a stronghold of Sunni insurgents since 2003 as well as a safe haven for Al Qaeda in Iraq militants. Local insurgents, however, rose against Al Qaeda last year and joined the U.S. military in the fight against the terror network.

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 army and security agencies, giving an edge to the insurgency there.

Reports on the number of casualties in the bombings varied.

Cmdr. Scott Rye, a U.S. military spokesman, said 12 people were killed in the attack and that 28 were wounded. Earlier, al-Moussawi said six people were killed and 26 wounded.

But an official at Azamiyah's al-Noaman hospital and an army official said seven people had been killed, and that 28 had been wounded. The police officer who said 14 had been killed said some casualties had also been taken to another hospital.

The witnesses and most Iraqi officials spoke on condition of anonymity for fear of reprisals or because they were not officially authorized to speak to media.

In an audiotape released Dec. 29, bin Laden warned Iraq's Sunni Arabs against joining the "awakening" groups or participating in any unity government.

U.S. military spokesman Maj. Gen. Kevin Bergner said last week the attacks were the "clearest indication" that the foreign-led Al Qaeda was worried about losing the support of its fellow Sunni Arabs.

A number of insurgent groups are thought to have switched allegiances and joined the "awakening" movement. There are more than 70,000 men in about 300 such groups being bankrolled by the U.S. around Iraq, and the number is expected to grow.

In eastern Baghdad, a roadside bomb detonated near a technology university, killing four people, including a student, and wounding 11 others, a police officer said.

In Baghdad's Jadriyah neighborhood, two roadside bombs went off minutes apart, killing one civilian and wounding four other people, including three policeman, another police officer said. Both spoke on condition of anonymity because they were not authorized to release information.

The first bomb detonated near Baghdad University on the southeastern bank of the Tigris river, killing a passing driver and wounding his passenger. About three minutes later, the officer said, another bomb went off near a police patrol that had responded to the first bombing, injuring three policemen.

In eastern Baghdad's Sinaa neighborhood, a parked car bomb exploded near a restaurant in the afternoon, police said, wounding seven people.

A mortar attack targeted a police station in eastern Baghdad killed two civilians and wounded five others, a police officer and medical worker said.

Meanwhile, the U.S. military said one of its unmanned surveillance planes crashed in Iskandariyah, 50 kilometers (30 miles) south of Baghdad. Local police said it went down in a school yard, but that no one was hurt. The military said an investigation into what caused the crash was under way.

Separately, a fire broke out in Iraq's largest oil refinery in Beiji, 250 kilometers (155 miles) north of Baghdad, killing at least one person and injuring nine, police said. Engineers said maintenance work had been carried out on units producing liquid gas and petrol, and that the blaze was believed to have been caused by a technical problem as one unit began operating again.

Beiji serves as a key transfer point for crude oil being exported out of Iraq.