The area accounts for 50 percent to 60 percent of insurgent attacks in Iraq, the Los Angeles Times said.

U.S. soldiers moved in the darkness early Tuesday across a bridge where only days before the insurgents of al-Qaida in Iraq had left a severed human head, the man identified as a U.S. collaborator with a black marker across his forehead, the Times said.

The 4,000 U.S. and Iraqi troops were backed by air support, but before the offensive began there were reports that 50 to 60 insurgent leaders had fled. The Times said that followed a longstanding pattern -- as U.S. and Iraqi troops move in, insurgents abandon their weapons and blend into the civilian population, only to emerge elsewhere.

Bombs struck two U.S. vehicles as the troops moved in, and troops had to bridge a road that had been cut.

The Times said it was only the latest in a series of operations in the area.