A second day of fierce fighting has broken out in southern and central parts of Iraq. A two-month-old truce between followers of a radical Shia cleric and coalition troops appears to have evaporated. In the city of Najaf, the U.S. military estimates it has killed 300 militants over the past two days. Heavy fighting has also been raging in the streets of an impoverished Baghdad neighborhood, where Health Ministry officials say at least 19 people have been killed since Thursday.

Prolonged gun battles raged in the streets of the Baghdad slum known as Sadr City. Followers of the radical cleric, Moqtada al-Sadr, sealed off most of the streets leading into the neighborhood, in some cases using explosive devices as deadly roadblocks. Mr. al-Sadr's militia is known as the Mahdi army, and fired rocket-propelled grenades at U.S. and Iraqi troops.

On Thursday, Moqtada al-Sadr called on his followers to rise up and rebel against American troops. But late in the day, he appeared to back down, saying he was willing to observe the cease-fire that has kept the relative peace in Iraq's Shia-dominated areas since early June.

On Friday, several of his spokesmen sent mixed messages, with one saying the cleric wanted to reinstate the truce, and another saying he had declared America the enemy and urged his followers to fight on.

In Sadr City, a Baghdad slum named for Moqtada al-Sadr's respected father, a 65-year-old sheikh who gave his name as Abu Akeel and his occupation as mujahedin for the Mahdi army, indicated that the militants intend to fight on, regardless.

"No, this is not because of Mr. al-Sadr," he said. "This is natural. We have to fight. It is our moral duty to defend our country." He says Moqtada al-Sadr has not called for jihad or holy war. He blames the governor of Najaf and the interior minister for provoking the fighting.

The Iraqi government says it will not tolerate militia activity. A spokesman for interim Prime Minister Iyad Allawi said the militias are considered criminal and terrorist organizations. On Thursday, Interior Minister Falah al-Naqib delivered much the same message.

"We are not going to negotiations. We are going to fight those militias, and we have enough power and enough strength to stop and kick those people out, out of the country," he said.

Battles also flared Friday in the holy city of Najaf, an al-Sadr stronghold where the unrest started early Thursday. Clouds of black smoke could be seen hanging over parts of the city. U.S. military helicopters attacked a Mahdi army hideout in a cemetery near the shrine of Imam Ali, one of the holiest sites in Shi'ite Islam.

The violence continued unabated, as the most revered Shia cleric in Iraq, Grand Ayatollah Ali al-Sistani, left the country for medical care. He flew to London to receive treatment for a heart condition. Mr. al-Sistani is a moderate and very influential leader, and some observers worry that his absence at this tense time could make it harder to bring the situation under control.