U.S. and Iraqi troops have intensified their offensive in the northern city of Mosul to retake control of police stations and other government buildings seized by gunmen during the last week.

American helicopters and war planes provided cover as troops struck isolated pockets of resistance. Mosul's bridges are closed, and a curfew remains in effect from 4 p.m. to 6 a.m. (local time).

Many insurgents are believed to have fled to Mosul before U.S. forces began an offensive against the militant stronghold of Fallujah last week.

Meanwhile, in Fallujah, U.S. General George Casey said the week-long operation had been "a very substantial victory."

Much of the city sustained heavy damage during the offensive, and local authorities spent the day collecting the dead from streets, alleyways and buildings.

The U.S. military is investigating an incident in Fallujah, after a videotape broadcast Monday showed a Marine shooting dead at close range a wounded, apparently unarmed militant.

An American journalist (Kevin Sites) embedded with the Marines filmed the shooting Saturday.

The reporter said five militants inside a mosque had been wounded in clashes with Marines on Friday. After the fighting ended, the Marines left the wounded rebels behind, expecting other troops to pick them up.

But the men were never cared for.

On Saturday, a different group of Marines came upon the wounded militants, which led to the videotaped incident. The Marines said they did not know at the time whether the rebels were armed.