The steadily mounting U.S. death toll reflects an insurgency that has not buckled despite facing off against a military super power, analysts said.

"They've been very adaptive and resilient," said defense analyst Ted Carpenter of the Cato Institute think tank. "That's one of the chief problems that an intervening force faces in any counterinsurgency war. You're fighting on the adversary's home turf and essentially all the enemy has to do is to out-wait the intervening power."

Military medical experts say the U.S. death toll would be even higher if not for advances in medical care and body armor that keep alive badly wounded troops who would have died in previous wars.

They point to: advances in body armor, with torso armor better protecting the chest and abdomen, heart and lungs and helmets better protecting the brain; improved in-country surgical capabilities allowing patients to be stabilized and quickly flown out of Iraq; and better prepared battlefield medics.

The deadliest month of the war was November 2004, when 137 U.S. troops died in a month when U.S. forces conducted a fierce offensive in the city of Falluja in the western Anbar province to deny Sunni Muslim insurgents a safe haven.

U.S. fatalities had dropped in five straight months from last November through this March, as insurgents appeared to focus more of their violence on Iraqi civilians and American-trained Iraqi government security forces.

But the U.S. death tolls in April and May were above average, and the Pentagon has acknowledged a recent surge in insurgent violence.