BAGHDAD (AP) -- U.S. warplanes carried out at least two airstrikes overnight in Basra for the first time since clashes between Shiite militias and Iraqi security forces erupted in the southern oil port this week, a British official said Friday.
Shiite militants also clashed with government forces for a fourth day in Iraq's oil-rich south and sporadic fighting broke out in Baghdad, despite a weekend curfew in the capital.
Prime Minister Nouri al-Maliki has promised "no retreat" in the fight against militias in Basra despite growing anger among followers of radical Shiite cleric Muqtada al-Sadr and his Mahdi Army.
The crackdown has intensified Sadrist anger over recent raids and detentions. They say U.S. and Iraqi forces have taken advantage of their seven-month-old cease-fire to target the movement.
Al-Sadr on Thursday called for a political solution to the burgeoning crisis and an end to the "shedding of Iraqi blood." But the statement, released by a close aide, stopped short of ordering his Mahdi Army militia to halt attacks.
A British military spokesman in Basra said U.S. warplanes carried out at least two airstrikes overnight in Iraq's southern oil port.
Maj. Tom Holloway says jets have been providing air support since clashes between Shiite militias and Iraqi forces erupted in the southern oil port on Monday, but it was the first time bombs have been dropped.
Iraqis have been of control of security since the British withdrew last December but Britain maintain troops there to provide assistance when needed.
In Baghdad, a U.S. helicopter also fired a Hellfire missile during fighting in the Baghdad's militia stronghold of Sadr City early Friday, killing four gunmen, military spokesman Lt. Col. Steve Stover said.
Ground forces called for the airstrike after coming under small-arms fire while clearing a main supply route at 4:10 a.m., he added.
Iraqi police and hospital officials in Sadr City said five civilians were killed and four others wounded in the attack.
The strikes underscore the risks that the U.S. and its allies in Iraq could be drawn into an internal Shiite conflict that has threatened to unravel al-Sadr's cease-fire and spark a new cycle of violence after months of relative calm.
In political developments, the main Shiite bloc in parliament said it would not attend an emergency session called for Friday to find ways to end fighting between government forces and militiamen in southern Iraq.
Deputy parliamentary speaker Khalid al-Attiyah, also a member of the United Iraqi Alliance, said the events in the south are a law and order issue, not legislative.
The bloc has been in contact with its Kurdish allies to boycott Friday's session too, which would prevent a quorum, he said.
It was not immediately clear whether house speaker Mahmoud al-Mashhadani, a Sunni, would still attempt to convene a session.
Amid the crisis, the prime minister has decided to skip this weekend's Arab summit, officials said. Vice President Adel Abdul-Mahdi will attend the meeting in Syria instead, according to Laith Shobar, an adviser to the Shiite vice president.
The campaign to rid Basra, Iraq's second-largest city, 340 miles southeast of Baghdad, of lawless gangs and Shiite militias - some believed tied to nearby Iran - is a major test for the Shiite leader and for the Iraqi military.
The ability of Iraqi leaders and security forces to control situations like this one is key to U.S. hopes of withdrawing its forces from the country.
The prime minister put his credibility on the line by flying down to Basra on Monday and issuing a weekend deadline for the surrender of Mahdi Army militiamen loyal to al-Sadr.
But the Basra offensive has faced fierce resistance and the security operation has triggered a violent response among al-Sadr's followers in Baghdad and cities throughout the Shiite heartland of southern Iraq.
At least 12 militia fighters were killed and seven others wounded in fighting in Mahmoudiya, according to an Iraqi army official.
The local office of radical Shiite cleric Muqtada al-Sadr, meanwhile, claimed 15 Iraqi soldiers had been captured, including two officers, in the city, about 20 miles south of the capital.
Fierce fighting in the Mahdi Army stronghold of Nasiriyah also killed at least four people, including two policemen and two civilians, and wounded 14, an officer said, adding that the clashes had spread to other parts of the city.
Two Iraqi security forces also were killed and three wounded in Kut, police said.
The security officials all spoke on condition of anonymity because they weren't authorized to release the information.
Rockets or mortars also were lobbed at a U.S. facility in the southern city of Hillah, although no casualties were reported, the military said.
Al-Maliki's office also announced Friday that it has given residents in Basra until April 8 to turn over "heavy and medium-size weapons" in return for unspecified monetary compensation.
The deadline is separate from the three-day ultimatum for gunmen to surrender their arms and renounce violence or face harsher measures, which expires later Friday, government adviser Sadiq al-Rikabi said.
The move instead appeared to be aimed at noncombatants who may have weapons like machine-guns and grenade launchers either for smuggling purposes or to sell to militants or criminal gangs.
The government also announced a days-old curfew in Basra would be loosened to allow people to move around in the city from 6 a.m. to 6 p.m. to facilitate shopping and other necessary tasks. It also called on local agencies to use the time to help residents, who have complained of food shortages and other problems amid the chaos.
Despite the order, an Associated Press staffer said he and his family were fired upon by Iraqi security forces when they tried to leave their house.
Sporadic fighting was reported in predominantly Shiite areas in eastern Baghdad despite a curfew banning unauthorized movement in the capital was imposed from 11 p.m. Thursday to 5 a.m. Sunday.
Purported Mahdi Army gunmen abducted three policemen with their weapons and vehicle in one area and clashes erupted between militiamen and U.S.-Iraqi troops in another, according to police.
The U.S. military did not immediately comment on the latest reports but said 26 militants during operations Thursday in mostly Shiite areas in Baghdad.