BAGHDAD — U.S. forces stepped deeper into the Iraqi government's fight to cripple Shiite militias on Friday, launching airstrikes in the southern city of Basra and firing a missile into the main Shiite stronghold in Baghdad.
The American support came as Iraqi troops struggled against strong resistance in Basra and retaliation elsewhere in Shiite areas — including more salvos of rockets and mortars into the U.S.-protected Green Zone in Baghdad.
It was the first time American jets have been called to attack militia positions since Iraqi ground forces launched an operation Tuesday to clear Basra of the armed groups that have effectively ruled the streets of the country's second-largest city for nearly three years.
In Washington, President Bush said the battle against Shiite extremists presents "a defining moment in the history of Iraq" and a "necessary part of the development of a free society."
The United States has called the Basra campaign an important test of Iraq's ability to handle its security affairs. But setbacks in the battle could increasingly draw in American forces, worried that a sustained fight — and the backlash in Baghdad and elsewhere — could wipe away security gains of recent months.
Late Friday, one militia barrage slammed into the headquarters of the Basra police command, triggering a huge fire and explosions when one of the rounds struck a gasoline tanker, police officials said.
Earlier, U.S. jets struck a building housing militia fighters and blasted a mortar team firing on Iraqi forces, Maj. Tim Holloway, a British military spokesman, said without further details.
The crackdown in Basra has provoked a violent reaction — especially from the Mahdi Army of anti-American Shiite cleric Muqtada al-Sadr. His followers accuse rival Shiite parties in the government of trying to crush their movement before provincial elections this fall.
Their anger has led to a sharp increase in attacks against American troops in Shiite areas after months of relative calm after Sadr declared a unilateral cease-fire last August.
Prime Minister Nouri al-Maliki, a Shiite who once maintained close ties to Sadr, has put his personal prestige on the line in the Basra crackdown, flying to the city five days ago to assume personal command of the operation there.
Maliki has vowed there would be "no retreat" in Basra, the nation's commercial center and headquarters of the vital oil industry.