Fallujah's centre stormed

American forces thrust into the heart of Fallujah today, fighting fierce street battles and conducting house-to-house searches.

On the second day of a major assault to retake the city from Islamic militants, troops burst through the suburbs and were pressing on towards rebel strongholds in the north-west.

With soldiers taking up positions in the heart of Fallujah, witnesses reported at least one massive explosion had rocked the area. There had earlier been reports that rebels had deployed up to 100 cars packed with explosives to use as suicide bombs.

The US military said the operation was going "smoothly". Troops met lighter than expected resistance in Jolan, a

Sunni-militant held warren

of alleys in north-eastern Fallujah where the assault began. The US military said three troops had been killed and another 14 wounded in the area in the past 12 hours.

Heavy street clashes were raging in other northern areas amid fierce bursts of gunfire, residents said. At least two American tanks were engulfed in flames, according to witnesses. There was no confirmation of casualties. Witnesses earlier said one US helicopter was shot down by a groundlaunched rocket and a Kiowa helicopter flying over southeast Fallujah was fired on but the injured pilot managed to return to the US base.

Colonel Michael Formica, commander of the 1st Cavalry Division's 2nd Brigade, said that a security cordon around the city will be tightened to ensure insurgents dressed in civilian clothing do not slip out.

"My concern now is only one - not to allow any enemy to escape. I do not want these guys to get out of here. I want them killed or captured as they flee," he said.

Up to 15,000 US troops and 5,000 Iraqi soldiers are battling in the city that has become the major sanctuary for Islamic extremists who fought marines to a standstill last April.

The offensive is considered the most important military effort to re-establish government control over Sunni strongholds west of Baghdad before elections in January.

Progress on Operation Phantom Fury had started slowly in many areas with the most advanced units pushing about a kilometre into the city by dawn.

Insurgent forces, up to 6,000 strong, used simple but effective tactics honed last April to frustrate the troops through the night. To prevent their gun muzzle flashes being spotted, fighters stood back from windows as they fired from buildings and used brilliant red and blue flares to blind the Americans' sensitive night-vision equipment when cornered.

Amid the battle, loudspeakers at mosques were proclaiming "Prepare for jihad!" and "God is great!".

The Americans responded by driving trucks around the streets blaring out a mix of heavy metal tracks by AC/DC, cavalry charge themes and sonic blasts designed to disorientate rebel fighters. Many US units were pinned down by small arms fire, advancing only after they had spotted the rebel positions and called in artillery barrages or airstrikes to flatten entire buildings.

Terrified civilians stayed inside. "There are heavy air strikes everywhere and heavy clashes in the Jolan district," one resident said. Many fled the city of 300,000 before the offensive and it is not clear how many remain.

A clinic in central Fallujah that had been receiving casualties, was reportedly destroyed with some medical staff and patients killed.