American troops have been involved in prolonged street-fighting after launching one of the biggest military offensives since last year's assault on Fallujah.
US warplanes dropped 500lb bombs and helicopter gunships made repeated strafing sorties at the town of Husaybah near the Syrian border.
Dozens of insurgents were killed in the fighting, American commanders said, without their forces suffering any fatalities. However there were reports of large numbers of civilian casualties and a prominent Sunni leader accused the US and the Iraqi government of carrying out a deliberate "killing operation".
More than 3,500 American and Iraqi forces are engaged in Operation Steel Curtain in the western province of Anbar, where Abu Musab al-Zarqawi's al-Qa'ida in Iraq group is said to have bases and supply lines into Syria. Two previous large-scale offensives had failed to stop attacks by insurgents in the area, and yesterday large parts of Husaybah were empty, with many of its 30,000 residents leaving after apparently being forewarned about the attack.
But US and Iraqi troops came under sustained fire from insurgents using Kalashnikov rifles, machine-guns and rocket-propelled grenades and only a small part of the town had been secured by nightfall.
An American officer, Captain Conlon Carabine, of the Sixth Marine Regiment, said: "We met more resistance than we expected. I thought they were planning more of a defensive posture."
Another officer, Colonel Stephen Davis, added, while ordering Abrams tanks to blast away with their heavy-calibre guns: "I got bombs, he got bombs. I got more bombs that he got. It's a very primal fight. We don't do a lot of hearts and minds out here because it is irrelevant."
American officers said air strikes were called in to destroy suicide car bombs heading for their forces, and searches had revealed caches of arms, land-mines and explosives.
In Baghdad a senior Sunni politician, Saleh al-Mutlaq, who heads the Council for Dialogue, an umbrella group of political, religious and tribal groups, said: "American forces accompanied by what is called the Iraqi army and National Guard are conducting a destructive and killing operation of secure cities and villages on the pretext that they hide terrorists."
US officials said the operation was being carried out to try to establish security in the region and stop insurgents crossing from Syria. Husaybah is thought to be a main staging post for jihadists who enter Iraq through desert towns along the Euphrates river.
American commanders said they are due to go through every one of the 4,000 buildings in the town and surrounding areas, but many of them are suspected to have been booby trapped by insurgents.
Elsewhere, three American soldiers were killed in separate incidents over the weekend, bringing the number of American dead to 2,045 since the start of the Iraq war in March 2003.
Opposition to the American offensive grew among Sunni leaders yesterday. In Baghdad, Mohsen Abdul-Hamid, head of the Iraqi Islamic Party, a moderate Sunni Arab bloc, said: "We reject all military operations directed against civilian targets because such acts lead to the killing of innocent people and the destruction of towns and cities."Reuse content