Security has been stepped up across Iraq following the attack in the southern town of Nasiriya which killed up to 31 people. Codenamed Iron Hammer, the operation has been described as a far tougher approach by the U.S.

In Tikrit U.S. helicopter gunships killed seven Iraqis and arrested at least four suspected of involvement in last week’s downing of a Black Hawk helicopter that killed all six Americans on board.

In Basra, Coalition Provisional Authority staff have been confined to their headquarters until Saturday evening after the threat of a possible attack.

In Baghdad, troops fired mortars and artillery at a former Republican Guard compound near the international airport. In a separate incident Iraqi guerillas attacked a convoy north of the capital, killing a U.S. civilian contractor and wounding another American. The U.S. military also is looking into the reported disappearance of an American contractor, Kirk von Ackermann who went missing last month while driving between Tikrit and Kirkuk.