US-led coalition forces killed at least 20 militants after coming under heavy attack from Shia gunmen in southern Iraq's Maysan province early today, the US military said.
An official in the office of anti-American Shia cleric Moqtada al-Sadr said 17 members of the Mehdi Army had been killed and 45 wounded.
That would mark one of the deadliest clashes between US-led forces and the Mehdi Army since a major security crackdown began in Iraq four months ago.
Local officials said British and Iraqi forces were involved in the raids in Amara and the nearby town of Majjar al-Kabir, which the US military said targeted militants smuggling weapons from neighbouring Iran into Iraq.
Maysan has a long border with Shia Iran, which Washington accuses of arming, funding and training Shia militias in Iraq. Tehran denies the charge.
During the operations, coalition forces came under heavy small-arms fire and rocket-propelled grenade attacks in both places, the US military said.