Up to 5,000 US and Iraqi troops have poured into the city of Samarra to try to overcome rebel forces there.
The US says rebels have tried to set up roadblocks in the city, which is on the main road from Baghdad to north Iraq.
It also said US forces had to diffuse a possible car bomb. Samarra residents said US warplanes bombed the city as tanks rolled through the streets.
Samarra is one of several areas in Iraq which have fallen under the control of rebel forces in recent months.