Iraq's top Shi'ite Muslim cleric flew to London Friday for treatment for a heart condition. In central and southern Iraq, U.S. forces clashed for a second day with militia loyal to another Shi'ite cleric. The U.S. military says some 300 militants have been killed in two days of fighting.

An aide to Grand Ayatollah Ali al-Sistani calls it a "health crisis," which caused the ayatollah to cancel all his meetings several days ago. Few details were made public about his illness, but an aide did not describe the condition as life threatening.

The 73-year-old cleric is considered a moderate influence over Iraq's Shi'ites who constitute a majority of the population.

A radical Shi'ite cleric, Moqtada al-Sadr, resumed his fight against U.S. forces this week. The violence is the worst since a shaky truce ended a two-month rebellion earlier this year against the U.S.-led occupation.

The U.S. military says several hundred militants have been killed in clashes in a predominantly Shi'ite neighborhood of Baghdad and in the holy city of Najaf south of Baghdad.

During Friday prayers, a cleric close to Moqtada al-Sadr blamed U.S. forces for the resurgence of violence in Najaf. A gunman stood guard behind the cleric as he spoke.

A spokesman for Moqtada al-Sadr hinted on Friday the fighters might be ready to negotiate another truce. Sheik Mahmoud al-Sudani says the militia, known as the Mahdi army, does not want an escalation. He says they are willing to establish stability in Iraq.

The governor of Najaf called on U.S. forces for help after the militants attacked a police station. He has called the Mahdi army criminals, and issued an ultimatum for them to leave the city in 24 hours.

The U.S. administration has insisted the terrorist attacks would not shake its resolve to keep U.S. forces in Iraq for as long as the they are needed. President Bush on Friday reiterated that commitment.

"The ultimate success of our venture in Iraq, which is a free and democratic county will depend on how quickly we can help the Iraqis defend themselves," he said.

Iraq's interim government has pledged to disband all independent militias, including the Mahdi army.

"We are not going to negotiations," said Interior Minister Falah al-Naqib. "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."

In other news, four Lebanese truck drivers have been abducted in Iraq. Earlier in the week, several Jordanian hostages were freed in a raid on their hideout. Turkish officials are still trying to free three citizens being held hostage in Iraq. Two others are reported missing.