BAGHDAD - A suicide bomber detonated an explosive belt Thursday inside a municipal government building west of Baghdad, killing at least 12 people attending a meeting of tribal sheiks, police said.

Col. Fawzi Fraih, civil defense director of Anbar province, said the sheiks were members of a group opposed to al-Qaida in Iraq and were meeting with Americans when the attack occurred in Karmah, about 20 miles west of Baghdad.

The U.S. military would not confirm whether Americans were inside the building during the attack, the third against a municipal government meeting in Iraq this week.

Police said the bomber entered the building through a back door, but it was unclear how he managed to evade security for the meeting, which drew community leaders in the town where Sunnis have turned against al-Qaida in Iraq.

U.S. and Iraqi troops rushed to the bombing site and sealed off the area, local residents said by telephone.

The media office for Anbar province said the dead included the town's administrative director and at least two chiefs of major Sunni tribes in the area.

The attack occurred only days before U.S. troops are to hand over security responsibility for Anbar to the Iraqis, marking a major milestone in the campaign to lower the U.S. profile in an area that had once been center-stage of the war.

Both Sunni and Shiite extremists appear determined to try to undermine efforts to build government institutions at the local level.

Ten people, including four Americans, were killed Tuesday in a bombing in a municipal council office in the Shiite area of Sadr City in Baghdad.

Two Americans were shot dead and four wounded Monday when a disgruntled official opened fire as they left a municipal building in Salman Pak about 15 miles south of the capital.

The U.S. military says violence in Iraq has dropped to its lowest level in more than four years, but attacks are continuing as Sunni and Shiite extremists try to regroup and undermine security gains.