In Washington, Mr. Bush strongly defended Prime Minister Nuri Kamal al-Maliki on Friday at what he called a “defining moment” for the Baghdad government, saying the United States supported the Iraqi offensive and would provide any military assistance that was sought.

“It’s still a dangerous, fragile situation in Iraq,” Mr. Bush said when asked whether the renewed fighting might draw Americans in more deeply and disrupt plans to withdraw more American combat troops in the next few months. He said his top military commanders and national security advisers, who briefed him on Iraq earlier in the week, are “all aiming to make sure that we have enough of a presence to make sure that we’re successful in Iraq.”

In Basra, the conflict has put the city into a state of virtual siege, with electricity and drinkable water cut off in most neighborhoods. Militiamen sabotaged a major oil pipeline in the city, according to a Western official. And fighters in the Mahdi Army militia tried to raid the Central Bank, but were repulsed.

F-18 fighter jets dropped cannon rounds on a militia stronghold and on a mortar team that was attacking Iraqi forces in Basra. The airstrikes, one at 9 p.m. Thursday and a second after midnight, were made at the request of the Iraqi Army, said Maj. Tom Holloway, a spokesman for the British Army.

Major Holloway said that British and American planes had been conducting surveillance runs over Basra since the fighting began to support the Iraqi military, but that this was the first time they had entered active fighting. “I think the point here is actually that the Iraqis are capable, they are strong and they have been engaging successfully,” Major Holloway said.

A Western official said negotiations between Iraqi government leaders and militia leaders in Basra had begun even as gunmen clashed. The official said that British and American aircraft were providing aerial support and that a small number of American ground troops had also entered Basra for the first time in years, mainly to monitor the performance of Iraqi forces.

The need to call in the American-led forces raised questions about the Iraqi Army’s ability to wage a successful campaign on its own. Witnesses have said that large parts of Basra  perhaps as much as 50 to 70 percent  are still controlled by Shiite militias.