A buildup of U.S. military force continues near the Iraqi city of Najaf, where Shiite militias are being warned to drop their weapons, or risk an all out assault by the U.S. led coalition. A top United Nations envoy in Iraq is warning the current level of violence will have to diminish, if elections are going to be held in the country by early next year.

More than 2,000 U.S. troops surrounding the Shi'ite holy city of Najaf are holding their fire, waiting to see if an agreement can be reached with cleric Moqtada al-Sadr to end an uprising by thousands of his militia followers. U.S. military commanders have vowed to kill or capture the cleric, who is wanted in connection with the murder of a rival Shi'ite figure a year ago.

"And fighting continues in the town of Fallujah, despite a cease-fire," he said. "At least four more U.S. Marines have been reported killed in Iraq, making April now the deadliest month for American forces there since the end of major combat was declared last May. "

In Baghdad, visiting United Nations envoy Lakhdar Brahimi is warning violence has to be checked, if Iraqis are going to hold elections by next January.

"The security situation has to improve significantly for these elections to take place in an acceptable environment," he said.

The U.N. envoy has been in Iraq since early this month, part of efforts to determine what political entity will take power after June 30, when the U.S.-led military occupation plans to hand political sovereignty back to Iraqis.

But as the fighting continues, so do attacks against foreign civilians. Four bodies - not yet identified - have been found near where an American fuel convoy was attacked last week. At least 40 foreigners continue to be held hostage by groups opposed to the U.S.-led occupation, and the attacks have reached such a level that Russia, which has long kept civilian workers in Iraq, is planning to evacuate its citizens.

During a rare prime time news conference Tuesday night, President Bush rejected comparisons some critics are drawing between Iraq and the Vietnam war. But he prepared the nation for what could be a long military presence in Iraq, saying military commanders will be provided with whatever level of troops they need to maintain security.

"Troop strength now and in the future is determined by the situation on the ground. If additional forces are needed, I will send them," the president said.

Despite the upsurge in violence, the president reaffirmed his commitment to handing power back to Iraqis on June 30.