The US military is investigating a video clip that appeared on the internet purporting to show Western security contractors firing at Iraqi civilian vehicles, a spokesman said yesterday.

A company of the California Army National Guard has been placed on restricted duty amid allegations that members of the unit mistreated detainees in Iraq, US military officials said.

Insurgents in western Iraq killed five US Marines and sliced the heads off three Iraqis, part of a group of 21 men whose bodies were found dumped in the desert near the Syrian border after being gagged and shot.