A homicide bomber struck Thursday inside a municipal building west of Baghdad, killing at least 20 people at a meeting of tribal sheiks opposed to Al Qaeda, police said. The U.S. confirmed American casualties but gave no further details.

Another 18 people were killed and about 60 wounded in a car bombing Thursday near a government headquarters in the northern city of Mosul, officials said. The attacks were part of a spike in violence in Iraq after weeks of relative calm.

Col. Fawzi Fraih, civil defense director of Anbar province, said the sheiks were meeting with Americans when the attack occurred in the town of Karmah, about 20 miles west of Baghdad.

Local police Capt. Amir al-Jumaili said 20 Iraqis were killed and 20 others wounded.

The U.S. military said the attack resulted "in casualties, including coalition forces and local nationals" but gave no further details. American authorities said they suspected Al Qaeda in Iraq was behind the attack.

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.

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

The bomb in Mosul exploded between the government headquarters and a market, where the governor of surrounding Nineveh province, Duraid Kashmola, was inspecting damage from an earlier rocket attack, police said.

Kashmola said that 18 people had been killed and 60 wounded.

The attack in Karmah 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 (25 kilometers) 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.

Targets of those attacks have included local administrations which the U.S. and Iraqi authorities have been trying to shore up to maintain the security gains since last year.

Also Thursday, American troops killed two suspected Al Qaeda militants and captured 15, including two Egyptians, in raids Thursday in central and northern Iraq, the U.S. military said.

The two extremists were killed in Sharqat, about 135 miles north of Baghdad, after they refused to surrender to U.S. troops who had surrounded the building where the pair had taken refuge, the U.S. said in a statement.

One of the dead was identified as a militant cell leader who was the target of the raid, the U.S. said. Three people were taken into custody.

The two Egyptians were detained in Abu Ghraib on the western edge of Baghdad for allegedly helping mount homicide attacks in the area, the U.S. said. A third person was seized near Abu Ghraib for allegedly providing weapons and homicide vests to Sunni militants.

The other arrests occurred during raids west of Sinjar in northern Iraq, Mosul and the Bijar area between Mosul and Baghdad, the military said.

At least 10 American soldiers have been killed this week in a spike of violence. That has pushed the monthly death toll for American troops in Iraq to at least 26 — well below figures of last year but an increase over the 19 who died in May, the lowest monthly tally of the war.