Two US helicopter gunships attacked Shia militia positions in the holy city of Najaf on Sunday, hours after Iraq's government ordered the gunmen to lay down their weapons, a Reuters witness said.
The witness said the gunships fired missiles at positions near Najaf's ancient cemetery manned by the Mehdi Army militia loyal to radical cleric Moqtada al-Sadr.
It is the fourth consecutive day of fighting that has claimed hundreds of casualties.
Interim Prime Minister Mr Iyad Allawi, on a surprise visit to the city earlier in the day, had ruled out negotiations with the militia and demanded gunmen lay down their arms and quit Najaf.