The U.S. military said the air- and ground-offensive dubbed Operation Swarmer was aimed at clearing "a suspected insurgent operating area" northeast of Samarra and was expected to continue over several days. (Related video: More on Operation Swarmer)

Residents in the targeted area said there was a heavy U.S. and Iraqi troop presence in the area and large explosions could be heard in the distance. It was unclear if the blasts were due to fighting.

The military termed the operation the largest air assault since the 2003 invasion, but it was not clear if any U.S. aircraft opened fire during the operation or if there had been any insurgent resistance.

"More than 1,500 Iraqi and Coalition troops, over 200 tactical vehicles, and more than 50 aircraft participated in the operation," the military statement said.

It was not clear from the 101st Airborne's initial statement exactly what it meant by characterizing the attack as the largest air assault operation since the beginning of the Iraq war. It could refer to the number of aircraft involved or the amount of weaponry involved or some other measure of size.

There was no immediate word on whether any fighter jets or other fixed-wing warplanes had dropped bombs or fired missiles as part of the assault. Also left unsaid was how many of the 1,500 total troops involved are Iraqis.

U.S., Iraqis launch 'Operation Swarmer'