"But were the government that takes over to ask us to leave, we would leave," Straw said. Britain is the main force other than the United States in the U.S.-led military coalition that brought down Iraq's authoritarian government last year and is trying to restore calm in the aftermath.

Powell said he expected an American would continue as the commander of the coalition forces in Iraq, who would report up his chain of command to maintain military effectiveness. Also, a consultative process could be established so the U.S. commander and the American ambassador kept the Iraqi government informed of their activities, he said.

French officials are urging that the new Iraqi government be given the power to evict U.S. forces if it so chooses.

"There has to be a complete break with the past, with the Iraqi government replacing the coalition," said French Foreign Minister Michel Barnier.

Al-Sadr's militiamen attacked U.S.-led coalition headquarters in Nasiriyah, trapping international staff and some Italian journalists inside. Explosions and gunfire rocked Karbala, and al-Sadr's top aides threatened to unleash more attacks across the Shiite south and in Baghdad.

"We will fight and defend the holy shrines until our last breath," al-Sadr said in an interview broadcast late Friday by Al-Arabiya television, widely seen throughout the Middle East. "We are not controlling any holy shrine — we are defending these shrines."

Several large explosions and the roar of high-flying aircraft could be heard in Baghdad before dawn today. The U.S. command issued no statement, and the cause of the blasts was unknown.

The fighting around Najaf, the most important center of Shiite theology and scholarship, unnerved the country's Shiite majority, including members who have disavowed al-Sadr and worked with U.S. authorities.

Hamid al-Bayati, spokesman for a mainstream Shiite group represented on the U.S.-appointed Iraqi Governing Council, called the fighting a "big mistake" that could inflame sectarian passions. He urged both sides to mediate an end to the standoff.

At least four Iraqis were killed and 26 wounded Friday in Najaf, according to Haidar Raheem Naama, a hospital official. He said most were civilian. One coalition soldier was wounded, U.S. officials said.

At least three militiamen also were killed, and their coffins were brought to the Shrine of Imam Ali for family and friends to pray for their souls.

"America is the enemy of God," fighters shouted.

Explosions and heavy machine-gun fire rocked Najaf for hours, and bands of gunmen carrying assault rifles, rocket-propelled grenades and mortar tubes roamed the city. After a lull, sporadic firing resumed as night fell.