Hundreds of US soldiers have swept through the western Iraqi town of Haditha, searching homes and seizing suspected insurgents.
One man was arrested, apparently for owning too much ammunition
"About 15 [troops] came to my house and searched everything, looking for arms and asking if I knew any insurgents," an academic, Maher Dali, told Reuters.
At least 10 people are reported to have been killed in fierce gun battles.
The area, 200km (125 miles) north-west of Baghdad, has been the scene of frequent attacks on US-led troops.
An Associated Press report says helicopters dropped US marines near palm groves to block off one side of the town, while other troops established checkpoints and moved into the centre of the city on foot and in armoured vehicles.
"Right now there's a larger threat than should be in Haditha and we're here to tell them that they're not welcome," battalion commander Lt Col Lionel Urquhart told the news agency.
The US marines said six insurgents were killed in a battle in central Haditha, including one man identified as a cleric, and four died in separate incidents elsewhere.
Two US soldiers were injured in the fighting.
Residents said troops were searching houses and most people were too frightened to leave their homes.
One resident was witnessed being arrested by US marines for having too much ammunition for a licensed weapon.
The man was blindfolded and had a code number written on his forehead, as his mother and sisters pleaded with the troops and their Iraqi translator to release him.
It is the second major offensive in the western Anbar province to tackle the insurgency, which has claimed more than 600 lives since the new Iraqi government was formed nearly a month ago.
Operation Matador used similar numbers of troops earlier in May to attack suspected insurgents in towns close to the Syrian border.