Firemen wash a street after three U.S. soldiers and many Iraqis were killed in a blast Thursday in Dora neighborhood in Baghdad. The Iraqi capital has seen an uptick in attacks since April, as U.S. troops prepare to leave the country by the end of June and hand over more security operations to their Iraqi counterparts.

Baghdad ? Bombers struck in Baghdad and a northern city Thursday, killing three American soldiers and nearly two dozen Iraqis in a new spasm of violence that has taken at least 66 lives in two days.

A spate of attacks since April seems aimed at stoking sectarian tension and undermining public confidence only weeks before U.S. combat troops are due to leave Baghdad and other cities, handing security responsibility to Iraq’s security forces.

Although recent violence has not risen to levels of two years ago, it has fueled public unease over whether Iraq’s army and police can maintain the security gains since the 2007 U.S. troop surge.

Most of the attacks this year have been on Shiite targets, suggesting that al-Qaida and other Sunni Arab extremists are trying to rekindle sectarian fighting and undermine Prime Minister Nouri al-Maliki within his core Shiite constituency.

Attacks have accelerated since the Shiite-led government began cracking down on Sunni paramilitary groups that abandoned the insurgency and joined forces with the U.S. to augment security.

The deadliest blast Thursday occurred in an outdoor market in Baghdad’s southern Dora district, when a bomb exploded near an American foot patrol, U.S. and Iraqi officials said.

U.S. officials said three soldiers were killed and an undisclosed number of Americans were wounded. Iraqi police said 12 civilians also were killed and 25 wounded. The Iraqis spoke on condition of anonymity because they were not supposed to release the information.

Army Maj. David Shoupe said U.S. officials could not confirm Iraqi police and witness reports that the attack was carried out by a suicide bomber.

Khalil Hamza, a college student who lives near the market, said he noticed four passengers in a white car watching the Americans as they walked through the area. After the blast, he saw the car speed away with three men inside.

“Thick smoke filled the area and the shoppers were in panic,” he said. “Ambulances arrived and rescuers started to look for the dead and wounded through the smoke and amid cries for help from the wounded.”

Earlier Thursday, seven Sunni paramilitaries were killed and eight wounded when a suicide bomber detonated his explosives as they stood in line waiting to be paid at a military base in the northern city of Kirkuk, police Maj. Salam Zankana said.

Paramilitary groups, known as Awakening Councils or Sons of Iraq, have been frequently targeted by al-Qaida and other Sunni insurgent groups.

Also Thursday, a bomb exploded in a trash container inside a Baghdad police station, killing three policemen and wounding 19 others, an Iraqi police official said. The official spoke on condition of anonymity because he was not authorized to release the information.

The attacks came a day after a car bomb exploded near a group of restaurants in a Shiite neighborhood of northwest Baghdad, killing 41 people and wounding more than 70.

U.S. troops are due to leave Iraqi cities by the end of next month under terms of the U.S.-Iraq security agreement that took effect Jan. 1.

President Barack Obama plans to remove combat troops from the country by September 2010, with all U.S. forces out of Iraq by the end of 2011.