WASHINGTON — U.S.-led coalition forces would leave Iraq if a new interim government should ask them to, Secretary of State Colin Powell said Friday, but such a request is unlikely.

Powell said the United States believes that a U.N. resolution passed last year and Iraqi administrative law provide necessary authority for coalition forces to remain even beyond the scheduled June 30 handover of the government to Iraqis.

"We're there to support the Iraqi people and protect them and the new government," Powell said at a news conference with his counterparts from other Group of Eight nations preparing for an economic summit next month. "I have no doubt the new government will welcome our presence and am losing no sleep over whether they will ask us to stay."

But were the new government to say it could handle security, "then we would leave," Powell said.

While those discussions were taking place, American tanks made their deepest incursion yet Friday into Najaf, the stronghold of radical cleric Muqtada al-Sadr. Apparent gunfire slightly damaged one of the Islamic Shiites' holiest shrines, prompting calls for revenge and even suicide attacks.

Echoing Powell, L. Paul Bremer, the top U.S. administrator in Iraq, told a delegation from Iraq's Diyala province Friday that American forces would not stay where they were unwelcome.

"If the provisional government asks us to leave, we will leave," Bremer said, referring to the interim Iraqi administration due to take power June 30. "I don't think that will happen, but obviously we don't stay in countries where we're not welcome."

Undersecretary of State Marc Grossman had told the House International Relations Committee on Thursday that although it was unlikely, the Iraqi interim government could tell U.S. troops to leave. But Lt. Gen. Walter Sharp, who was also at the hearing, contradicted his statement, telling the panel that only an elected government could order a U.S. withdrawal.

White House spokesman Scott McClellan told reporters Friday that the Iraqi people still want help from the United States and coalition forces to provide security.

"Iraqi security forces are not fully equipped and trained to provide for their own security and defend their country against terrorists," McClellan said. "And so, after the transfer of sovereignty on June 30, we expect to continue to partner with the Iraqi forces to improve the security situation."

British Foreign Secretary Jack Straw said at the news conference with Powell that stability in Iraq would not be served by an abrupt withdrawal.

"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.

Four holes, each approximately 12 inches long and 8 inches wide, could be seen on the golden dome of the Imam Ali mosque, burial place of Imam Ali Ibn Abu Talib, the Prophet Muhammad's cousin and son-in-law and the Shiites' most revered saint.

The mosque, in the middle of Najaf, is about 100 miles south of Baghdad on a high desert plateau overlooking the world's largest cemetery.

Militia members blamed the Americans for the damage to the mosque, but Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq, said al-Sadr's men were probably responsible: "I can just tell you by the looks of where we were firing and where Muqtada's militia was firing, I would put my money that Muqtada caused it."

During the crackdown on al-Sadr's al-Mahdi Army, U.S. forces have been careful to avoid damaging shrines for fear of enraging Iraq's Shiite majority. They have attacked mosques where insurgents have set up fighting positions.

At a press conference in Baghdad, Kimmitt pointed to a map of Najaf and said a U.S. convoy might have been fired on from the cemetery as it moved near the shrine. If so, those rounds could have hit the shrine, he said.

Kimmitt accused the militia of using religious sites "much like human shields." He said American forces had not initiated the fighting but were responding to attacks by al-Sadr's gunmen.

That did little to assuage the anger of many Shiites in Najaf. By early evening, thousands gathered around the Imam Ali shrine to inspect the damage. Some shook their heads in disbelief. Others mumbled prayers.

"The Americans had better leave Iraq after this," said Jassim Mohammed. Another man, Abu Zahraa al-Daraji, added: "The Americans have crossed a red line."

Al-Sadr's aides called on their followers to rise up against the coalition. His representative in Nasiriyah, Sheik Aws al-Khafaji, threatened attacks on coalition forces there, most of whom are Italians.

After his threat, armed men attacked coalition headquarters in Nasiriyah, about 200 miles southeast of Baghdad. They fired at least five rocket-propelled grenades within a half hour as Italian troops and Filipino security guards fought back.

About 10 coalition staffers, including Italians, Americans and Britons along with 10 drivers and security guards were trapped in the building along with four Italian journalists, coalition officials said.

"It's an inferno," Maria Cuffaro, a journalist for Italy's state-run RAI network, said during a brief live report on Italian television late Friday. "We're all OK, if a bit shaken."

Explosions and gunfire also rocked another Shiite holy city, Karbala, as U.S. troops clashed with al-Sadr's militiamen. Shops were closed and residents stayed off the streets.

In Baghdad, aides to al-Sadr urged followers in Sadr City to travel to Najaf to reinforce the militia. Al-Sadr's representative in the southern city of Basra, Sheik Abdul-Sattar al-Bahadli, said he would form suicide squads to carry out attacks on coalition forces and urged residents to register for the squads starting Saturday.

And in the southern city of Amarah, al-Sadr aide Farqad al-Mousawi warned Iraqi police and civil defense corps members that they risked assassination if they helped U.S. soldiers fight al-Sadr's militia.

Japan's Kyodo News service reported shooting and an explosion late Friday in Samawah, a southern city where Japanese and Dutch troops are based. The shooting started after armed, masked al-Sadr supporters began sealing off downtown streets. One Iraqi security officer was killed, Kyodo said.

Al-Sadr launched an uprising against the coalition last month after U.S. officials announced he was wanted for the April 2003 murder of a cleric in Najaf. He lacks the spiritual stature of Grand Ayatollah Ali al-Husseini al-Sistani, and his confrontational tactics have exasperated moderate Shiites.

However, al-Sadr commands the support of thousands of mainly poor, urban Shiites who admire his father, a grand ayatollah who was killed by Saddam Hussein's agents. Al-Sadr has also capitalized on hostility toward the coalition following revelations of abuse of Iraqi prisoners by U.S. soldiers.

Despite the fighting, al-Sadr delivered a sermon at Friday prayers in Kufa, another holy city that lies six miles to the northeast of Najaf, as he has for the past four Fridays.

Al-Sadr described President Bush and British Prime Minister Tony Blair as "the heads of tyranny" and accused them of ignoring the suffering of Iraqis in coalition prisons while drawing attention to what he described as the "fabricated" case of Nicholas Berg, an American civilian who was beheaded by militants.

In other developments:

The U.S. Army announced criminal charges, including adultery, maltreatment of detainees and committing indecent acts, against Military Police Cpl. Charles A. Graner in the Abu Ghraib prisoner abuse scandal. He will be arraigned May 20. Three other military police face charges in the scandal.

The United States freed 293 Iraqi detainees from Abu Ghraib. The coalition periodically releases detainees from the prison, which now holds more than 3,000 prisoners.

A U.S. military supply convoy was attacked 25 miles north of Baghdad and one fuel tanker was destroyed. Iraqi youths danced and cheered around the burning vehicle as they displayed family photos, presumably the driver's.

Contributing: Hamza Hendawi