The first came in the afternoon, when Shiite militiamen opened fire on a U.S. patrol, and seven insurgents were killed. Hours later, a M1 tank was attacked with rocket-propelled grenades, triggering a heavy battle in which warplanes destroyed an anti-aircraft gun belonging to the militia, and 57 gunmen were killed, Kimmitt said.

Najaf hospitals listed 37 dead, all young men of fighting age, suggesting they may have been militiamen. Sheik Amer al-Husseini, an official at Sadr's office in Baghdad, said 25 were killed. He did not say how many of the casualties were militiamen.

Night video taken by the Associated Press Television News between Najaf and the nearby town of Kufa showed U.S. army helicopters flying low over smoke rising from an area in the distance amid flashes of gunfire.

An al-Sadr aide in Najaf, Mustaq al-Khafaji, accused Americans of trying to advance toward Kufa. "We will face the Americans whenever they show up," he said.

U.S. authorities have vowed to capture al-Sadr and uproot his militia, the al-Mahdi Army, which launched a bloody uprising at the beginning of April.

About 2,000 troops are deployed outside Najaf, but the military is having to tread carefully. Any action that even brings the possibility of harm to the sacred Imam Ali Shrine at its heart could turn the limited al-Sadr revolt into a widespread uprising by Iraq's Shiite majority.

Bremer heightened warnings about the reported stockpiling of weapons in "mosques, shrines and schools" in Najaf.

"The coalition certainly will not tolerate this situation," Bremer said in a statement to residents of Najaf.

About 200 soldiers on Monday moved into a base that Spanish forces are abandoning in Najaf.

In Madrid, Prime Minister Jose Luis Rodriguez Zapatero said Spain has completed the withdrawal of its troops, recalling his campaign pledge to bring them home unless the United Nations took military and political control of the occupation.

The Baghdad attack Tuesday killed a U.S. soldier and wounded another in the Shiite neighborhood of Sadr City, a stronghold of al-Sadr's Al-Mahdi Army militia, Kimmitt told reporters.

The death brought to 115 the number of U.S. troops killed in combat in the past 27 days — the same number of Americans killed during the two-month invasion of Iraq that toppled Saddam.

Meanwhile, Bulgarian President Georgi Parvanov said his country's troops were "not prepared" for the kind of fighting they are doing in Iraq and need "immediate and substantial military backup" from the coalition.

Speaking in the Bulgarian capital of Sofia after a visit to the 485-member contingent Sunday, Parvanov said he wants the troops be relocated to a new camp outside Karbala by June 30. Karbala has been the scene of recent heavy fighting by al-Sadr's followers.

British Prime Minister Tony Blair said Britain has no plans to send more troops to Iraq.

"The advice that we have now is that we have sufficient troops to do the job," Blair said at a news conference. Britain currently has 7,500 troops in southern Iraq.