Many people were killed or wounded by a US air strike called to support Iraqi forces in the town of Hilla south of Baghdad yesterday, Iraqi security sources said.

US forces confirmed the air strike and said they were not certain how many people had been killed but denied that there were large numbers of casualties. One police source said at least 11 people were killed and 18 wounded in the strike, launched after Iraqi security forces called for support following street battles with Shi'ite militia members in the city's Thawra neighbourhood.

Another police source said 29 people were killed and 39 were wounded. He said six houses were destroyed in the strikes which lasted for an hour late yesterday evening.

Two other security sources said the combined total of dead and wounded was in the dozens, although they were unable to give precise casualty figures. All of the sources spoke under condition they not be named, as is usual practice in Iraq.

Major Allayne Conway, spokesman for US forces south of Baghdad, said US helicopters had responded to a call for help from Swat special police units in Hilla.

"The Hilla Swat guys were on the ground. They were engaged. Our attack helicopters were called in. They engaged," she said.

"We're still checking how many enemy personnel were killed. The initial report I had was four."

Iraqi security forces have battled Shi'ite militia in several southern cities and Shi'ite areas of Baghdad for the past two days.

Meanwhile Iraqi Prime Minister Nuri al-Maliki ordered Shi'ite militiamen to surrender yesterday as his forces staged a crackdown on followers of powerful cleric Moqtada al-Sadr that threatened to wreck a ceasefire.

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

More than 80 people have been killed and hundreds wounded in the fighting, centred on the southern oil hub of Basra and spreading to Shi'ite parts of Baghdad where Sadr's followers hold sway and the towns of Hilla, Kut and Diwaniya in the south.

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