US and Iraqi troops are reported to be sweeping through the north-western Iraqi town of Tal Afar near the Syrian border in an anti-insurgency operation.
Bradley fighting vehicles patrolling the streets have been coming under small-arms fire, AP news agency quoted eyewitnesses as saying.
Tal Afar police chief Capt Amjad Hashim said about 20 suspected insurgents had been captured.
He said a US helicopter had come under fire and was forced to return to base.
US Army spokesman Sgt John Franzen said the helicopter had suffered "mechanical problems", but he was unaware if it had been hit by enemy fire.
Sgt Franzen said the offensive, which began at about dawn on Tuesday, was targeting "terror suspects" responsible for attacks on civilians, AP reports.
The operation comes a day after US and Iraqi officials met tribal chiefs in the town to discuss methods to tackle security problems and restore police and local government services there.
Asked about clashes in Tal Afar by a presenter on Arabic TV channel al-Jazeera, Iraq government security adviser Gen Wafiq al-Samarrai dismissed the question.
"Clashes can happen anywhere," he said.
The town is reported to have frequently served as a base for attacks on Iraqi and US forces and representatives of political parties participating in the new Iraqi government.