US forces killed almost 20 Iraqi militiamen in fighting overnight around the Shia holy city of Najaf. Two US soldiers were also killed in the skirmishes.
The US army said one 1st Armoured Division soldier was killed in an ambush and another when a grenade struck his tank.
The latest fighting comes on the fourth day of clashes since militia leader Moqtada al-Sadr offered a truce. Al-Sadr declared a ceasefire on Thursday, after pressure from the Shia religious and political establishment who are exasperated by two months of bloodshed between US forces and Sadr's Mehdi Army.
US commanders welcomed the offer to pull his forces off the streets but maintained their demands that he turn himself in on a murder charge and fully disband his militia.
A US army spokesman said the truce did not seem to apply Najaf's twin city of Kufa, although at least one local Mehdi Army squad commander said it did and accused US forces of provoking his men.
US tanks advanced into Kufa toward the main mosque and skirmished with Sadr's fighters based around it for about two hours around midnight (9 p.m. Irish time), residents said. US commanders have said they would be willing to wait several days to assess whether the ceasefire was holding, however.
Another soldier was killed in the region yesterday, bringing the total US combat death toll in Iraq to 591.