Operation Shank, involving 300 American Marines and 200 Iraqi soldiers, began on Friday, the latest in a series of operations in Anbar province, the heartland of anti-US fighting.
A US military statement said: "The purpose of the operation is to disrupt a terrorist group that utilises an area of Ramadi as its base for attacks on local Ramadi citizens, Iraqi and US military."
Fighters launched a brief assault in Ramadi on Thursday, firing mortar rounds and rockets at a US base and local government buildings.
Leaflets were distributed saying that al-Qaida in Iraq, the group led by Abu Musab al-Zarqawi, was taking control of Ramadi, the capital of Anbar.
But after a couple of hours, most of the fighters dispersed and the city appeared to return to relative calm.
The US military said reports of fighters taking control of the town were "completely unsubstantiated".
Town residents said there was no sign of the fighters on the streets on Friday.
Many moved to Ramadi after US forces crushed their base in nearby Falluja in November last year.
US and Iraqi forces launched a new operation in an area west of Ramadi on Wednesday, sending more than 2000 soldiers and marines into Hit and surrounding towns to track down fighters and try to restore security ahead of polls.
The US Marine Corps also announced on Friday that 10 marines were killed on Thursday by a roadside bomb near Falluja, 50km west of Baghdad. The announcement said a further 11 marines had been wounded in what was one of the deadliest attacks on American soldiers in recent months.
A statement said the men were from Regimental Combat Team 8, of the 2nd Marine Division. While on foot patrol, they were hit by an improvised explosive device made from several large artillery shells, the army said.
Seven of the wounded have returned to duty. The names of those killed were withheld pending notification of their relatives.