US tanks and Marines stormed into Falluja last night in a fierce ground assault to retake the Iraqi city from rebels.
Several tanks thrust into the city and guerrillas were putting up some resistance, Marine radio traffic showed.
Intense air strikes, artillery and mortar fire rained down on the city. Flares lit up the night sky as the Marines earlier unleashed a barrage of tank and machine gun fire on a nearby railway station, clearing the way for the ground assault on the city, which is 50 kilometres west of Baghdad.
The top US commander in Iraq predicted a "major confrontation" on the streets as a US-led force of as many as 15,000 troops pressed to retake control of the Sunni Muslim city.
Gen George Casey said that the offensive, which began on Sunday, was proceeding on schedule. He expected the insurgents to use home-made roadside bombs and car bombs as their "weapon of choice".
With as many as 15,000 troops participating in the siege, he predicted that insurgent defenders on the outer edges of the city would fall back into the centre for a "major confrontation" with US and allied Iraqi forces.
Gen Casey said that 50 to 70 per cent of the city's roughly 200,000 people had left, meaning there could be as many as 100,000 people still there.
He said the figure of 3,000 for the number of insurgents in Falluja was "in the ball-park".
"They have a range of weapons from AK-47s and machine guns, rocket-propelled grenades, some heavier anti-aircraft-type machine guns," he said.
"But the weapons of choice for them are going to be the improvised explosive devices [roadside bombs] and the car bombs. And all our intelligence is telling us that they lined some of the streets with the improvised explosive devices ... and they have also placed car bombs around the city," he added.
Some insurgents managed to slip away, he said, while others "have moved in".
The Iraqi Prime Minister, Mr Iyad Allawi, visited Iraqi troops at the main US base near Falluja a few hours before the main offensive began and told them they had to free the people of the city who had been "taken hostage" by insurgents. "Your job is to arrest the killers, but if you kill them then let it be."
At a news conference in Baghdad, Mr Allawi said: "We are determined to clean Falluja from the terrorists."
Intense fighting shook Falluja in the morning. F-16 fighters screamed across cloudy skies, dropping bombs that sent up clouds of black smoke.
When air attacks eased, artillery shells rained down. Cobra helicopters fired rockets and gunfire crackled as US forces peered through binoculars at guerrilla targets.
A hospital doctor in Falluja said 15 people had been killed and 20 wounded in the fighting.
The Sunni Muslim Clerics Association urged Iraqi security forces not to fight with US troops in Falluja and "to beware of making the grave mistake of invading Iraqi cities under the banner of forces who respect no religion or human rights".
A spokesman for Shia cleric Moqtada al-Sadr told Al Jazeera television yesterday: "We strongly condemn the planned attack by occupation forces against Falluja and therefore ask members of the Iraqi National Guard, army and police not to help occupying forces target the sons of Iraq in Falluja. They should not be tools in the hands of the occupation because it targets everybody."
A British Black Watch soldier was killed and two were injured - one seriously - in a suspected roadside bombing yesterday. Meanwhile, gunmen killed a US soldier in eastern Baghdad in a separate attack, the US military said last night.