NAJAF, Iraq (Reuters) - Militiamen loyal to radical Shi'ite cleric Moqtada al-Sadr fought gun battles with foreign troops in southern Iraq for a third day Tuesday and their leaders demanded the U.S.-led occupiers leave towns and cities.

The United States has said it will arrest Sadr, accused of leading a wave of confrontations with U.S.-led forces, who announced the deaths of seven more soldiers Tuesday.

The battles in Shi'ite Muslim areas represent a new front for occupying troops trying to pacify Iraq, though President Bush has insisted violence would not derail U.S. plans for a June 30 handover of sovereignty to an Iraqi government.

Fighting between Sadr's followers and Italian troops in Nassiriya killed about 15 Iraqis and a Ukrainian soldier was killed and six wounded in clashes near the town of Kut.

Sadr's militia, known as the Mehdi Army, has held violent protests and launched attacks in several Iraqi cities in the last three days to protest the detention of one of his advisers 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.

Sadr's supporters said they would fight until occupying troops left populated areas and prisoners were released.

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 echoed across Falluja, in the "Sunni triangle" where U.S. troops face daily attacks. The U.S. military said four Marines had been killed Monday in the volatile al-Anbar province, which includes Ramadi and Falluja.

"BUSH'S VIETNAM"

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

U.S. Democratic Senator Edward Kennedy, a key backer of Democrat John Kerry's election bid, said Iraq had become "George Bush's Vietnam," referring to the war that divided the United States and helped drive Lyndon Johnson from the presidency.

But Paul Bremer, Iraq's U.S. administrator, disagreed.

"...I don't even know where to start with that comparison," he told NBC's "Today" show. "I think it's completely inappropriate. There is really nothing in common with 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.

A total of 429 U.S. troops have been killed in action in Iraq since the invasion launched last March to topple Saddam Hussein, who had been accused of hiding weapons of mass destruction.

The 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.

The former U.N. weapons inspector who had been charged with finding Saddam's illegal weapons said Tuesday that the costs of the war outweighed the benefits of removing Saddam.

"It's positive that Saddam and his bloody regime is gone, but when one weighs the costs, it's clearly the negative aspects that dominate," Hans Blix told Danish newspaper Jyllands-Posten.

SHI'ITE UPRISING

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 resist any attempt to detain him.

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 three American soldiers had been killed in the Shi'ite Kadhimiya neighborhood of Baghdad in incidents Monday and Tuesday.

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 said 12 Italian soldiers were hurt.

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

The Marines have launched a major new mission to pacify Falluja, imposing a curfew and sealing off roads.