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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 fuelled 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.

Sami Ghayashi, 37, who was among the wounded in Kirkuk, said the local council members had been waiting three months to receive their salaries.

"While we were waiting at gate talking to one another a big explosion took place," he said from a hospital bed. "I saw several colleagues dead, among them my cousin. I have no idea how this suicide bomber got among us."

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 neighbourhood of northwest Baghdad, killing 41 people and wounding more than 70.

It was the capital's first major car bombing since May 6 and the deadliest in the city since twin car blasts killed 51 people in another Shiite neighbourhood, Sadr City, on April 29.

Despite those attacks, security in Baghdad remains much better than a few years ago, when the rumble of explosions reverberated daily through the city. In recent months, violence has been cyclical, with periods of calm punctuated by brief series of high-profile attacks.

Nonetheless, the limited attacks have stoked public anger over the failure of Iraqi security forces to prevent bombings, especially in areas where large numbers of civilians congregate.

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.

In wake of the recent attacks, parliament's defence and security committee plans to meet Sunday to review the security situation, a Kurdish member of the panel, Firyad Rawndouzi, said.

He blamed the violence on al-Qaida and Saddam Hussein's Sunni-dominated Baath party, saying they "have regrouped recently and were able to revive some of their cells."

"The information we have is that the Baath party is taking the lead in conducting attacks now and al-Qaida has retreated to second place," Rawndouzi said. "Some of our security forces have slacked recently after the improvement in the security situation and this has given some freedom to the terrorists to move."

Iraqi officials have maintained for months that remnants of Saddam's party and al-Qaida were co-operating to plan and carry out attacks, despite broad philosophic differences.