More than 4,000 US Marines and troops entered Fallujah today, beginning a massive ground assault against Sunni Muslim insurgents in the biggest military operation since last year's invasion.

The attack on the central Iraqi city started after Iyad Allawi, the acting Iraqi Prime Minister, announced that he had given the multinational force authority to "clean Fallujah from the terrorists" to allow free elections to take place as scheduled in January.

The assault has been dubbed Operation Phantom Fury. It was heralded by a crushing bombardment of the Askari neighbourhood that rose to a crescendo by this evening, with US planes dropping bombs around the clock and artillery pounding the city every few minutes with shells.

The first attack came from north of the city, where Marine Regimental Combat Team 1 - more than 4,000 Marines and Army troops, along with Iraqi allies - had been massed since last night.

Backed by the 1st Cavalry Division's tanks and armour, the Marines swarmed into the city's once teeming Jolan district, the warren-like heart of Fallujah where the insurgents are concentrated .

US artillery had pounded the district ahead of the attack, hoping to set off any roadside bombs and boobytraps planted by the insurgents to slow down the advance. At the same time, US aircraft and artillery blasted an insurgent mortar position in southern Fallujah. The Army said nine militants were believed killed.

General George Casey, the top US commander in Iraq, said this afternoon that the attack was proceeding "on schedule" with as many as 15,000 coalition troops involved in the siege.

In a conference call with reporters, General Casy said that the vast majority of Fallujah's 300,000 civilians had left the city. Some rebels had also managed to slip away, he said, while others "have moved in."

Casey described the Iraqi rebels as "an amorphous group of terrorists and insurgents" and said not one single group appeared to be in change.

Confirmation of the assault from Dr Allawi in Baghdad was seen largely as a political move to show that Iraqis and not Americans are calling the shots as the US-led coalition takes its largest military gamble since the invasion of Iraq.

Dr Allawi told a news conference that 38 insurgents were killed and four foreign terrorists, including two Moroccans, had been captured as the town hospital was taken in an opening attack overnight.

He also announced a curfew on both Fallujah and another rebel stronghold, Ramadi, said he was closing Baghdad airport for 48 hours, and announced that Iraq's borders with Jordan and Syria would be closed except for trucks carrying food.

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