BAGHDAD  —  Iraq's prime minister pledged Thursday to expand his crackdown on Shiite militias in Baghdad, despite a mixed performance so far against militants in the southern city of Basra.

The U.S. ambassador, meanwhile, said that despite a "boatload" of problems with the Basra operation, he was encouraged that the Shiite-led government was finally confronting extremists regardless of their religious affiliation.

Iraqi forces launched a major operation March 25 to rid Basra of Shiite militias and criminal gangs that had effectively ruled the city of 2 million people since 2005.

But the offensive stalled in the face of fierce resistance from the militiamen and an uprising across the Shiite south spearheaded by the Mahdi Army of anti-American cleric Muqtada al-Sadr.

Fighting eased Sunday when al-Sadr ordered his fighters to stand down under a deal brokered in Iran.

Nevertheless, Prime Minister Nouri al-Maliki, himself a Shiite, insisted that the campaign to reclaim Basra was on track and that he would soon go after "criminal gangs" in Baghdad and elsewhere.

Al-Maliki specified two Baghdad neighborhoods — Sadr City and Shula — where the Mahdi militia holds sway. Both neighborhoods remain under a vehicle ban imposed last week throughout Baghdad but which has been lifted elsewhere in the capital.

"We cannot remain silent about our people and families in Sadr City, Shula and other areas ... while they are held hostage by gangs that control them," al-Maliki said. "We must liberate these cities because we came into office to serve them."

Al-Maliki also said the government would spend $US100 million dollars to improve public services in Basra and create 25,000 jobs there — moves aimed at weaning away support for the militias as security forces revamp their tactics to combat the extremists.

Last week's offensive in Basra resulted in a dramatic spike in violence — including rocket and mortar attacks on the U.S.-controlled Green Zone in Baghdad.

The violence erupted as the two top American officials in Iraq were preparing to brief Congress on prospects for further U.S. troop cuts.

U.S. Ambassador Ryan Crocker, who will appear before Congress on Tuesday with top commander Gen. David Petraeus, said he was surprised at the way the Basra campaign unfolded.

"I had the understanding that this was going to be an effort to get down, show they were serious with additional forces, put the squeeze on, develop a full picture of conditions and then act accordingly," he told reporters Thursday. "I was not expecting, frankly, a major battle from Day One."

Still, Crocker said he was encouraged that the Iraqi government was willing to take on Shiite militias, some of which maintain close ties to major political parties in the national leadership.

"Were there problems? There were a boatload of problems, and they still have a long way to go," Crocker added.

In a statement Thursday, al-Sadr complained that although he had called on his militia to stop fighting, the army and police were continuing illegal arrests and attacks against his followers.

Al-Sadr blamed the attacks on "corrupt elements" and said if the government could not remove them, "we are ready to cooperate...to purge our army and police of such elements."

Although major fighting in the south eased last weekend, military operations are continuing in the Basra area.

U.S. and Iraqi officials have maintained that the crackdown was directed at criminals and renegade militiamen but not al-Sadr's political movement, which holds 30 of the 275 seats in the national parliament and is a major political force.

But the Sadrists believed the operation was aimed at weakening their movement before provincial elections this fall. Resistance was so fierce that the Iraqis had to call in U.S. jets and British tanks and artillery to help in the battle.

On Thursday, the U.S. military said an American F/A-18 fighter jet fired a missile at a house in Basra the night before after Iraqi soldiers came under small-arms fire. Two militants were killed in the airstrike, the U.S. said.

"Coalition forces are unaware of any civilians killed in the strike but are currently looking into the matter," the military said. Iraqi witnesses and officials said at least three civilians were among the dead.

"While we were preparing for evening prayer, U.S. aircraft bombed this house, we rushed to save survivors but in vain," a neighbor identified only as Haj Juwad told AP Television News. "The father, mother and a young boy were killed and three others were buried under rubble. We evacuated two people and one is still under the rubble."

Hospital officials, who have strong ties to the Sadrists, said three bodies had been received, including two men and an elderly woman, and two women were wounded in the strike. The officials spoke on condition of anonymity because they weren't authorized to release the information.

In northern Iraq, officials said a suicide car bomber attacked an Iraqi checkpoint west of the city late Wednesday, killing seven people, including a woman and a 5-year-old child, and wounding 12, according to the Iraqi army.

The U.S. military confirmed the Mosul attack but put the casualty toll at five dead and 19 wounded.

In Baghdad, one civilian was killed and 10 were wounded in a parked car bombing, while an Iraqi soldier was killed and three were wounded when a roadside bomb struck their patrol.

Three Iraqi civilians also were wounded in a U.S. airstrike targeting militants engaged in a gunbattle with U.S. ground forces near the southern Shiite city of Hillah, according to a military statement.