The deaths of as many as two dozen Iraqi civilians and an ensuing cover-up threaten to do more harm to US efforts in Iraq than even the Abu Ghraib prison scandal, a prominent congressman and war critic says.

A criminal investigation into the deaths of about two dozen Iraqi civilians points toward a conclusion that Marines committed unprovoked murders, a senior US defence official said today.

US MARINES forced Iraqi youngsters to kneel while troops discharged a weapon in a mock execution, used electric shocks on one prisoner and set fire to a puddle of solvent that burned one of them, according to US Navy documents.

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.

The Arab-American US Marine once feared beheaded in Iraq said he was thankful to be home and asked people to pray for all hostages.

A US marine who turned up in Beirut after being missing for 18 days from his base in Iraq was being flown to Germany today for debriefing and examination in a US military hospital.

The US Postal Service closed 11 Washington-area post offices today while authorities checked to see if anthrax was detected at a Navy office handling mail.

The US Postal Service closed its government mail offices in Washington while tests were carried out to determine whether anthrax was detected at the Navy site that handles letters and parcels for federal agencies.