(reuters) - U.S. forces killed 45 Shi'ite militiamen in Baghdad in fierce fighting that included a tank battle with dozens of gunmen who attacked a checkpoint under cover of a dust storm, military officials said on Monday.
Four U.S. soldiers were also killed in rocket or mortar attacks in the Iraqi capital on Monday, the military said.
The U.S. military said the checkpoint attack on Sunday night sparked the biggest battle in the city since Prime Minister Nuri al-Maliki cracked down on militias a month ago.
It said 22 gunmen died in the assault. Another 23 were killed in other battles since Sunday in and around the eastern Baghdad slum district of Sadr City, stronghold of anti-American Shi'ite cleric Moqtada al-Sadr.
The fighting showed some members of the cleric's Mehdi Army militia have apparently defied his order to observe a truce, raising questions about how much he controls his followers.
U.S. and Iraqi troops have been locked in a month of fighting with militiamen since Maliki, himself a Shi'ite, ordered the offensive in the southern oil city of Basra.
They have taken over about a quarter of Baghdad's Sadr City where Sadr's fighters held sway.
The Sunday attack was accompanied by multiple rocket attacks on the Green Zone government and diplomatic compound in central Baghdad. Militants fired more rockets and mortars on Monday. Among the casualties were three U.S. soldiers who were killed in eastern Baghdad and one soldier in the city's west, the military said. It gave no further details.
U.S. troops fired tank guns tanks to repel the raid on the checkpoint. Other fighters fled.
Maliki says the crackdown is designed to disarm militias, but Sadr's followers see it as an effort to sideline the mass movement before provincial elections in October.
The movement, competing in local polls for the first time, could do well at the expense of Shi'ite parties backing Maliki.
ATTACK HELICOPTERS GROUNDED
The attacks in Baghdad have coincided with dust storms and were launched despite a call by Sadr on Friday for calm.
The dust storms have grounded Apache attack helicopters, the main weapon used by U.S. forces to hunt militants firing rockets and gunmen roaming the streets.
"We are aware of the rise in attacks and how they correspond to bad weather," said a U.S. military spokesman in Baghdad.
At the United Nations, Washington accused Iran of fuelling the clashes, saying it was training and supplying weapons to militias -- which Tehran denies.
"The recent clashes between criminal militia elements and Iraqi government forces in Basra and Baghdad have highlighted Iran's destabilising influence and actions," Washington's U.N. ambassador Zalmay Khalilzad told the Security Council.
After initial setbacks, Maliki's offensive in Basra seems to have been successful, driving militiamen off the streets.
Sadr City has proven a tougher challenge.
"Militias have infiltrated the state, the society," Deputy Prime Minister Barham Salih told Reuters in an interview.
"This cannot end without disarming these militias and dismantling them (or our) ... society will know no peace."
But Salih said a full-scale military assault in Sadr City, where 2 million people live, would risk too many civilian lives.
President Jalal Talabani, a Kurd, met members of Sadr's political faction and parliamentary leaders on Monday in an effort to resolve the impasse.
Sadr has not appeared in public in a year and the U.S. military says he is living in neighbouring Shi'ite Iran, where he is believed to be taking advanced Islamic studies.
Sadr launched two uprisings against U.S. forces in 2004.
His movement then entered politics and backed Maliki's rise to power in 2006. But Sadr split with Maliki a year ago when the prime minister refused to set a timetable for the withdrawal of U.S. troops from Iraq.
He has veered between open confrontation and peaceful politics in the five years since the U.S. invasion of Iraq, while remaining steadfastly anti-American and building a following of millions of poor Shi'ites.