BAGHDAD - A female bomber blew herself up yesterday in front of a building used by former Sunni Arab insurgents now siding with U.S. forces, one of two suicide attackers who killed at least 24 other people in Diyala province during the day.

It was the latest in a string of bombings targeting groups dubbed "concerned local citizens," or CLCs, that aid American efforts to combat the insurgency. Gunmen also ambushed and killed a Sunni tribal leader and his four guards who had been working with the Americans in the northern village of Rabia near the Syrian border, police said.

The U.S. military has employed more than 50,000 of these volunteers, including some former insurgents, to help police their areas in central and northern Iraq. They are credited with helping to drive out Sunni militants from Anbar province and parts of Baghdad, where attacks have dropped noticeably since the arrival of about 28,500 additional U.S. forces this year. But violence has flared in regions north of the capital, where many of the militants are believed to have fled.

Women rarely carry out suicide bombings in Iraq, but yesterday's attack was the second such bombing in Diyala in less than two weeks. On Nov. 27, a woman blew herself up near a U.S. patrol, wounding seven American troops and five Iraqi civilians near the provincial capital of Baqubah.

Yesterday, the second female bomber struck about 9:30 a.m. on a busy street in Muqdadiya, an ethnically and religiously mixed city about 60 miles northeast of Baghdad that remains a hotbed. Police said that 16 people were killed and 27 injured, but the U.S. military put the figure at 12 dead and 17 wounded.

The woman targeted a safe house used by members of the 1920 Revolution Brigades, an insurgent group that once fought alongside the Sunni militant group al-Qaeda in Iraq under the banner of the Islamic State of Iraq.

An Iraqi security official identified the bomber as Suhailah Hussein Chlayeb, a woman believed to be in her late 40s or early 50s. Three of her sons had been killed by the Iraqi army. The official spoke on condition of anonymity because he was not authorized to address the media.

North of Muqdadiya, an Iraqi army convoy was ambushed by gunmen firing AK-47s and rocket-propelled grenades, killing six soldiers and five volunteers, said Maj. Peggy Kageleiry, a U.S. military spokeswoman.

About 20 miles away, another suicide bomber drove his explosives-laden car into a checkpoint on the road between Delli Abbas and Mansouriyat al-Jibal, killing seven Iraqi soldiers and three additional volunteers, according to Iraqi security forces. In other violence yesterday, oil gushed into the Tigris River after an explosion damaged a pipeline supplying crude from the northern city of Kirkuk to the refinery in Baiji, about 125 miles north of Baghdad.

A dark cloud hung over the spot where the pipeline burst into flames, according to an official with the Facilities Protection Service in Kirkuk, which secures government installations. Officials suspended the flow of crude from Kirkuk to the refinery until they could repair the damage.

One person was killed and another injured in a drive-by shooting in Kirkuk, police said. A farmer was gunned down in a nearby village. In Baghdad, police recovered three bullet-riddled bodies, apparent victims of sectarian killings. The toll was far lower than the 30 or more bodies that were recovered daily before the U.S. troop buildup began early this year.