US Navy SEALs are stepping up their investigation into recently-published photographs that show freshly-captured prisoners and grinning commandos sitting or lying on hooded and handcuffed detainees in Iraq.

GUNMEN ambushed a bus full of Iraqis working for the US military, killing 17 civilians and wounding 13 in Tikrit yesterday, while a car bomb and a gun attack killed four members of the Iraqi security forces elsewhere in northern Iraq.

The US military has launched a criminal investigation into photographs that appear to show US Navy SEAL commandos in Iraq sitting on hooded and handcuffed detainees, it emerged today.