FALLUJAH: US troops last night stormed into Fallujah in a fierce ground assault to retake the Iraqi city from rebels.Several tanks thrust into the city after intense air strikes, artillery and mortar fire rained down during the day in an attack dubbed Operation Phantom Fury.

Flares lit up the night sky as the Marines earlier unleashed a barrage of tank and machinegun fire on a nearby railway station, clearing the way for the more the 15,000 troops to launch an all-out ground assault today.

US Defence Secretary Donald Rumsfeld and his commanders in Iraq predicted a "major confrontation" that would "take time".

"We are determined to clean Fallujah from the terrorists," Prime Minister Iyad Allawi said in Baghdad before visiting troops at the main US base near Fallujah. "Your job is to arrest the killers but if you kill them then let it be."

Throughout the day, F-16 fighters screamed across cloudy skies, dropping bombs that sent up clouds of black smoke. When air attacks eased, artillery shells rained down.

More than 38 militants were believed to have been killed yesterday while a hospital doctor said another 15 civilians had been killed and 20 wounded in the fighting.

With the offensive shaping up, Al Qaeda ally Abu Musab Al Zarqawi called on Muslims to take up arms against America.