U.S. military spokesman Major General William Caldwell said the number of car bombs in Baghdad, both detonated and defused, hit their highest level of the year last week and that bombs reported in general were "also at an all-time high".

U.S. and Iraqi forces have mounted a major military operation in the past two months against militants in Baghdad.

For the second time in two days, four U.S. soldiers were killed in a single incident around Baghdad, this time in what appears to have been a substantial skirmish involving mortars or rockets and gunfire to the northwest. It took the death toll in four days of the month to 15 around Baghdad and 22 in total.