A U.S. helicopter fired a Hellfire missile during fighting in a Shiite militia stronghold of Baghdad on Friday, while U.S. pilots conducted their first airstrikes on the southern city of Basra to aid Iraqi forces battling powerful Shiite militias.

At least four were killed in the U.S. airstrike in Baghdad's Sadr City neighborhood — a center for the Mahdi Army militia of anti-American cleric Muqtada al-Sadr. The U.S. military said the missile strike killed four militants, although Iraqi officials said the dead were civilians.

Ground forces called for the airstrike in Sadr City after coming under small-arms fire while clearing a main supply route before dawn, U.S. military spokesman Lt. Col. Steve Stover said.

He said four gunmen were killed, but Iraqi police and hospital officials said five civilians died and four others were wounded in the attack.

U.S. troops also fought militants on the ground in Sadr City and targeted rocket and mortar teams elsewhere in the capital, another spokesman Maj. Mark Cheadle said.

"We have conducted direct engagements from the ground and air in and around Sadr City when attacked or positively identified terrorists," he said.

The American air support marked a sharp escalation in the fight — so far led by Iraqi security forces — to cripple armed factions that the Pentagon accuses of links to Iran. It also suggested that the four-day-old ground offensives by Iraqi troops and police are struggling against strong militia resistance.

President Bush said Friday that the flare up in violence presents "a defining moment in the history of Iraq."

"It's going to take awhile, but it's a necessary part of the development of a free society," Bush said at a White House news conference with visiting Australian Prime Minister Kevin Rudd. At the same time, the president said the situation in Iraq remains "dangerous and fragile."

Washington has called the campaign an important test of Iraq's ability to handle its own security affairs. But setbacks in the battles could increasingly draw in American forces, worried that a sustained fight could wipe away many of the security gains in recent months.

American jets also dropped bombs overnight in Basra in the first use of U.S. air power in the southern oil port since the Iraqi government launched a crackdown against Shiite militias there earlier this week.

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Defying a curfew in Baghdad, extremists lobbed more rockets or mortars against the U.S.-protected Green Zone, which has come under steady barrages this week.

At least two rounds Friday struck the nearby offices of Sunni Vice President Tariq al-Hashemi, killing two guards and wounding four, his daughter Lubna said.

Thick black smoke rose into the sky in the latest of a week of attacks that have prompted the State Department to order all personnel at the embassy to stay in reinforced structures.

The Green Zone houses the U.S. Embassy and much of the Iraqi government and has come under fire with regularity since Easter Sunday.

At least two American civilians have been killed in the Green Zone attacks. Several others have been injured, some seriously, U.S. officials have said. Residents are holed up at home after a weekend curfew was imposed.

In Basra, the U.S. jets dropped bombs on a mortar team and a militia stronghold, said Maj. Tom Holloway, a British military spokesman. He did not have information about casualties.

The strikes came as tensions rose among followers of al-Sadr — angry over a crackdown that has threatened to unravel a militia cease-fire and spark a new cycle of violence after months of relative calm in Iraq.

The situation in Basra remained tense ahead of a Saturday deadline for gunmen to surrender their weapons and renounce violence. Masked militia fighters moved around freely in a southwestern neighborhood and there was little traffic, according to Associated Press Television News footage.

Prime Minister Nouri al-Maliki has insisted the fight is targeting criminal gangs in Basra, not al-Sadr's movement, and he has promised "no retreat."

But the crackdown has intensified Sadrist anger over recent raids and detentions by U.S. and Iraqi forces followers say have taken advantage of their seven-month-old cease-fire.

Al-Sadr on Thursday called for a political solution to the burgeoning crisis and an end to the "shedding of Iraqi blood" but stopped short of ordering his Mahdi Army militia to halt attacks in a statement released by an aide.

Another al-Sadr representative called al-Maliki "a hypocrite" during a Friday sermon calling for an end to military operations and the release of Sadrist detainees.

"He imprisoned and displaced thousands of Iraqi people under the name of democracy. He is killing the citizens in the south of Iraq," Sheik Jalil al-Sarghi said, referring to al-Maliki as U.S. helicopters buzzed over the office where the prayer service was held.

Parliament on Friday set up a committee led by its Sunni Arab speaker to try to mediate an end to the deadly clashes in southern Iraq, a member of the committee said.

Bassem Sharif of the Fadhila party said the decision was made during an emergency session, which lasted about two hours. An aide to Speaker Mahmoud al-Mashhadani said 78 lawmakers attended the session. There was no quorum, aide Jabbar al-Mashhadani said.

The house's largest Shiite bloc, the United Iraqi Alliance, boycotted the session, arguing the crackdown in the south is a question of law and order, not legislative.

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.

Helicopters on Friday dropped leaflets calling on residents to help the government in its fight "to rid Basra of outlaws."

Al-Maliki's office also announced 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 announced Wednesday for gunmen to surrender their arms and renounce violence or face harsher measures, 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.

At least 22 people — six civilians, four Iraqi security forces and 12 militants — were killed Friday in fierce fighting in the southern cities of Mahmoudiya, Nasiriyah and Kut, according to reports from police and army officials who spoke on condition of anonymity because they weren't authorized to release the information.

The U.S. military did not immediately comment on the latest reports but said 26 militants died during operations Thursday in mostly Shiite areas in Baghdad.