Iraqi Prime Minister Nuri al-Maliki ordered Shi'ite militiamen to surrender on Wednesday as a crackdown on followers of powerful cleric Moqtada al-Sadr spread across southern towns leaving a ceasefire in tatters.

Sadr, whose truce last year was praised by U.S. forces for curbing violence, called for talks to end the crackdown on his followers, the biggest military operation that Iraqi forces have undertaken without U.S. or British combat units.

Scores of people have been killed and hundreds wounded in the fighting, which began in the southern oil hub of Basra and spread to Shi'ite parts of Baghdad where Sadr's followers hold sway and the towns of Hilla, Kut and Diwaniya in the south.

Maliki, in Basra to oversee the campaign there, said fighters would be spared if they surrendered within 72 hours.

The assault is a chance for his government to prove it can impose its will and allow American forces to withdraw. But it also runs a risk of unleashing more violence after a year that saw security in Iraq improve dramatically.

"We have been living for the last hours in hell. We have spent most of the time hiding under the staircase," said Basra resident Faris Hayder, 28. "We haven't seen anything like this since the foreign troops arrived in 2003."

Battles which began on Tuesday in Basra resumed with heavy gunfire and explosions. A health official said 40 people had been killed and 200 wounded in the city by Wednesday morning.

A Reuters correspondent in Kut, 170 km (105 miles) south of Baghdad, heard gunfire and mortar impacts and saw buildings and cars aflame. Police said at least 18 people died in clashes there, including a baby girl.

In Hilla, several Iraqi security sources spoke of large-scale casualties after a U.S. air strike called to help Iraqi police fighting militiamen. U.S. forces confirmed the helicopter strike but denied there were large numbers killed.

In the capital, a health official said 14 people were killed and more than 140 wounded in clashes in the Sadr City slum.

Mortar bombs in other parts of the city killed nine people and wounded dozens, including three American civilians in the fortified Green Zone diplomatic and government compound. Two American soldiers died of bullet wounds.

A roadside bomb struck a U.S. patrol on a main road through Sadr City late on Wednesday and troops were cordoning off the area, a U.S. spokesman said. He had no details of casualties.

Iraqi forces also reported clashes in other mainly Shi'ite districts with a strong Sadr presence.

Such a big Iraqi operation would have been impossible a year ago, showing how far Iraqi forces have come, said U.S. military spokesman Major-General Kevin Bergner: "These are Iraqi decisions, they are Iraqi government forces and these are Iraqi leaders implementing and directing these decisions."

U.S. and British backing was limited to air support and teams of mentors embedded with Iraqi officers, Bergner said.

WITHDRAWAL PLANS

Washington aims to bring 20,000 of its 160,000 troops home by July after a build-up of troops improved security last year. U.S. Democratic candidates who aim to succeed President George W. Bush next January are calling for a faster withdrawal.

But violence has increased in the past few months and Iraqi forces have yet to show they can tackle militants on their own.

Sadr, a young, anti-American cleric, helped install Maliki in power after an election in 2005 but later broke with him. His followers, known as the Mehdi Army, have feuded with other Shi'ite groups seen as influential in Maliki's government.

Sadr declared a ceasefire last August, winning praise at the time from U.S. commanders for helping to reduce violence, although they say "rogue" Mehdi Army units outside Sadr's control have fought on with support from Iran.

Despite the violence, Sadr aides said the cleric's truce was still formally in place, a negotiating posture that could be useful for Sadr in coming days.

Sadr's followers have taken to the streets demonstrating against Maliki's government and forcing schools, universities and shops to close. On Tuesday he said he would call a "civil revolt" if attacks on his followers did not stop.

The head of Sadr's office in Basra, Harith al-Ithari, said the movement was negotiating with Maliki to end the fighting.

"There are ongoing negotiations with the prime minister. Maliki asked to meet Sadr officials in Basra," he told Reuters.

Another top aide, Hassan al-Zargani, read to Reuters what he said was a statement from Sadr calling on Maliki to leave Basra and appoint a delegation to hold talks.

Sadr has long been a thorn for rival Shi'ite groups in Maliki's circle, said Iraqi political analyst Hazem al-Nuaeimi.

"There is a need to minimise the Sadrists' strength and influence and to draw the lines before they get any stronger."

British forces, which patrolled Basra for nearly five years, withdrew to a base outside the city in December and were not involved in the fighting. A British military spokesman said the Iraqi assault was expected to last two to three more days.

An official with Iraq's Southern Oil Company said production in the Basra area which produces 80 percent of Iraq's exports could be disrupted if fighting lasted more than three days.