The city was calm but tense after intense US air and artillery bombardments on purported rebel safehouses left at least eight people dead and 23 wounded, according to medics at Fallujah's main hospital.

The operation followed Prime Minister Iyad Allawi's threat to invade the city unless its people handed over suspected Al-Qaeda operative, Abu Mussab al-Zarqawi, Iraq's most wanted man.

Troops, including one US army and one US marine infantry battalion, with tanks and Iraqi special forces, joined the overnight push on Fallujah, the epicenter of the Iraqi insurgency.

"Units are pushing forward... Their mission is to disrupt the enemy's ability to conduct terror attacks in this area of operation, specifically in the city of Fallujah... They'll do whatever it takes to accomplish that," Marine spokesman Lyle Gilbert told AFP.

It was the biggest deployment of US ground forces since last April when marines and insurgents battled in Fallujah, leaving hundreds dead. That campaign ended in an inconclusive stand-off.

Gilbert would not say if any forces had entered the rebel bastion but called it a major operation. The ground offensive followed a barrage of air and artilley strikes since Thursday.

Planes began bombing what the US military termed suspected hideouts of Zarqawi at 1 pm (1000 GMT) on Thursday as talks to avert a large-scale military showdown between Fallujah locals and the Iraqi government unraveled.

On Wednesday, Allawi told the Sunni Muslim stronghold to turn over Zarqawi, who has a 25-million-dollar price on his head, or face a military invasion.

"We have asked Fallujah residents to turn over Zarqawi and his group. If they don't do it, we are ready for major operations in Fallujah," Allawi said.

The US military trumpeted the barrage, which included some artillery strikes, as a bold new offensive upsetting a five-month-long stalemate in Fallujah, Zarqawi's suspected stronghold west of Baghdad.

"Iraqis and elements of the First Marine Expeditionary Force increased security operations in and near Fallujah tonight in order to disrupt preparations for terrorist attacks by anti-Iraqi Forces," the military said in a statement Thursday.

"The status quo in certain cities in Iraq is unacceptable. This operation puts the anti-Iraqi Forces (US military terminology for insurgents) in Fallujah on notice."

The last air strikes occurred at 2:38 am (2338 GMT Thursday) as the US military bombed two more suspected safefouses of Zarqawi, the man blamed for many of the suicide bombs and beheadings across Iraq.

"Operations included a range of weapon systems including armored vehicles, artillery, helicopters and strike aircraft," the statement said.

Earlier on Thursday, a delegation of city elders and leaders pulled out from talks with the government, protesting at Allawi's warning of invasion.

"We were taken aback by Allawi's comments... since there was no mention of Zarqawi during the talks," said one delegate.

"Allawi and his government will bear the responsibility of the spilling of Muslim blood in Fallujah."

He said delegates were close to reaching a breakthrough in talks that would allow Iraqi forces to return to the city before Allawi imposed "impossible conditions."

"Basically he was telling us that he did not want to negotiate, so we suspended the talks from our end," he said.

Since April, Fallujah has been a virtual no-go zone for American soldiers.

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