NAJAF, Iraq (Reuters) - Militiamen loyal to radical Shi'ite cleric Moqtada al-Sadr fought gun battles with foreign troops in southern Iraq Tuesday, with their leaders vowing to resist the U.S.-led occupation.

The United States has vowed to arrest Sadr, accused of leading a wave of bloody confrontations with U.S.-led forces.

But his supporters vowed Tuesday to fight until occupying troops left populated areas and prisoners were released.

Fighting between his followers and Italian troops in Nassiriya killed around 15 Iraqis and in the town of Kut witnesses said two Ukrainian armored vehicles were in flames.

Sadr's militia, known as the Mehdi Army, has held violent protests and launched attacks in several Iraqi cities in recent days to protest the detention of one of his aides and the closure of a militant newspaper by U.S.-led authorities.

"This insurrection shows that the Iraqi people are not satisfied with the occupation and they will not accept oppression," said a statement from the cleric, which an aide read at a news conference in the Shi'ite holy city of Najaf.

The U.S. army said three more American soldiers had been killed in Shi'ite areas of Baghdad.

Heavy fighting was also reported in the Sunni Muslim towns of Falluja and Ramadi, west of Baghdad, as U.S. Marines mounted a major operation to root out guerrillas. Residents said gunfire and blasts were echoing across Falluja.

NEW FRONT

U.S. troops have long faced daily attacks in the "Sunni triangle," including Falluja. But the fighting in Shi'ite Muslim areas represents a new front for occupying troops trying to pacify the country ahead of a June 30 handover of sovereignty to an Iraqi government.

President Bush has insisted the violence by Sadr's supporters would not derail Washington's plans for Iraq.

But a new opinion poll as Bush campaigns for November re-election showed support among U.S. voters for his handling of Iraq had fallen to a new low of 40 percent -- down 19 points since mid-January.

The U.S.-led administration said Monday an arrest warrant had been issued several months ago for Sadr in connection with the murder of another Shi'ite cleric last year.

Sadr's supporters said he was in his Najaf office and pledged to fight any attempt to detain him.

A Pew Research Center poll which reported the slump in approval for Bush's handling of Iraq also found 44 percent of Americans wanted U.S. troops withdrawn from the country.

U.S. Democratic Senator Edward Kennedy, a key backer of Democrat John Kerry's bid to unseat Bush in November, said Iraq had become "George Bush's Vietnam."

A senior U.S. army official said the Pentagon was looking at sending more troops to Iraq if the situation deteriorated, but believed it already had enough soldiers in place.

SHI'ITE UPRISING

Fighting has raged between Sadr's supporters and foreign troops since clashes in Baghdad and Najaf Sunday killed at least 48 Iraqis, eight U.S. soldiers and one Salvadoran soldier.

The U.S. army said two soldiers had been killed in the Shi'ite Kadhimiya neighborhood of Baghdad Monday and another in the same area Tuesday. All three attacks involved fighters armed with rocket-propelled grenades.

A Health Ministry spokesman said that in Baghdad 66 Iraqis had been killed and 317 wounded in the violence that began on Sunday. The impoverished Sadr City district has seen some of the heaviest clashes with U.S. troops.

In Nassiriya, 375 km (235 miles) southeast of Baghdad, clashes between Italian troops and Sadr's militiamen erupted before dawn as 500 Italian soldiers mounted an operation to dislodge fighters in control of key bridges in the town.

Paola della Casa, a spokeswoman for the U.S.-led Coalition Provisional Authority in the area, told Italy's Ansa news agency that about 15 Iraqis had been killed. An Italian officer in the town told Reuters that 12 Italian soldiers were wounded.

"We launched a large-scale operation this morning to restore public order to the town after two days of civil unrest," Major Simone Schiavone said. "The city was divided in two, with the bridges under their control. We had to go in and sort out the situation before it deteriorated."

Clashes were also reported in the southern towns of Amara, where British troops are stationed, and in Kut.

The U.S. military said four Marines had been killed on Monday in the volatile al-Anbar province, which includes Ramadi and Falluja. The Marines have launched a major new mission to pacify Falluja, imposing a curfew and sealing off roads.

The mission -- "Operation Vigilant Resolve" -- follows the killing and mutilation of four U.S. contractors in the town last week. A cheering crowd set the bodies ablaze and hacked them up.