Shia cleric Muqtada al-Sadr Wednesday urged his supporters to keep fighting Americans as U.S. soldiers killed 25 members of his militia during fighting in Karbala.

In his first news conference since his Mehdi Army militia took control of government and police offices in the cities of Karbala, Najaf and Kufa a month ago, al-Sadr urged his supporters to resist an American invasion. The cities are home to some of the holiest sites in Shia Islam.

"We are prepared for any American escalation and we expect one," said al-Sadr.

U.S. forces have been camped out around the cities throughout the standoff, but have avoided a full-out assault because of the religious importance of the cities.

Al-Sadr compared the fight in Iraq to the war in Vietnam.

"Let [me] remind you of Vietnam ... The means of victory that are available to us are much more than what the Vietnamese had," said al-Sadr.

Earlier on Wednesday, U.S. forces used tanks and helicopters in an assault on Karbala. American officials say at least 25 Iraqi fighters were killed.

They say seven coalition soldiers were injured.

Heavy damage has been reported in the city. A mosque al-Sadr used as an office was destroyed, seven hotels were on fire and most of the city's central market was destroyed.

On Tuesday, Iraqi tribal leaders said al-Sadr would call off his militia if the U.S. postpones a legal case against him and establishes an Iraqi force to patrol the city.

In early 2004, an Iraqi judge issued an arrest warrant for al-Sadr on charges relating to the murder of Abdul Majid al-Khoei, a senior Shia leader.