Iraq's top Shiite cleric warned the U.S. military against entering the holy cities of Karbala and Najaf as U.S. troops clashed with Shiite militiamen on Friday. Wanted Shiite leader Muqtada al-Sadr (search) said negotiations in his standoff with the United States were near collapse.

Al-Sadr militiamen attacked American soldiers outside the city of Kufa (search), neighboring Najaf (search). During clashes Friday, large explosions were seen by the river in a sparsely populated area on the edge of Kufa. Five civilians caught in the crossfire were killed and 14 wounded, hospital officials said.

Some 2,500 U.S. soldiers are deployed outside Najaf vowing to kill or capture al-Sadr and dismantle his al-Mahdi Army militia.

Iraq's top Shiite cleric, Grand Ayatollah Ali al-Husseini al-Sistani (search), warned of a strong Shiite response if U.S. forces enter the holy cities of Najaf or Karbala to capture al-Sadr.

The two cities are a "red line," Mahdi al-Karbala'i, al-Sistani's representative in Karbala, said during a sermon. "We are calling for peaceful solutions, but if the coalition forces are to cross the red line, then will take a different stronger position."

Al-Sistani, a moderate who has opposed anti-U.S. violence, holds enormous influence among Iraq's Shiite majority.

Al-Sadr slipped out of Najaf to Kufa and deliver the Friday prayer sermon at the main mosque.

"I am ready to meet martyrdom for the sake of Iraq," al-Sadr said, speaking with a white coffin shrowd over his shoulders — a symbol of his readiness to die. A crowd of worshippers chanted his name.

An al-Sadr spokesman said negotiations that Iraqi politicians have been mediating between the Americans and al-Sadr were close to collapse.

"I believe that the mediation will not continue for a long. They will reach a dead end today or tomorrow. There are no results from these negotiations and these negotiations could collapse," said Sheik Fuad al-Tarafi.

He said that according to the mediators, the Americans had made demands that were "impossible to fulfill," apparently that al-Sadr be handed over.