The skies above Fallujah burned red as operation Phantom Fury began with an aerial bombardment and a major ground offensive, after a go-ahead from Iraq's Prime Minister Iyad Allawi, said an AFP journalist embedded with the military.

US marines stormed the Jolan district in the northwest of Fallujah -- a notorious stronghold of rebels holed-up in the city -- while another unit took the Iraqi city's train station, a marine officer said.

A large-scale operation to retake Fallujah itself will begin Tuesday, Iraqi Defence Minister Sheikh Hazem Shaalan said. "We've called it Operation Dawn. God willing, it's going to be a new, happy dawn for the people of Fallujah."

Allawi gave the formal green light to launch the offensive and also visited some of the 12,000 troops taking part ahead of the battle at a US camp just outside the city, west of Baghdad.

"The people of Fallujah have been taken hostage just like the people of Samarra and you need to free them from their (the insurgents') grip," he told the soldiers.

"Your job is to arrest the killers but if you kill them then let it be," he said, warning the soldiers not to harm civilians.

Allawi earlier announced a stringent package of security measures to protect Iraq during the attempt to recapture the city, including a curfew in Fallujah and the closure of Baghdad's international airport.

In earlier skirmishes, multinational forces seized a hospital and two bridges on the western edge of the city.

Clashes with the insurgents holed up in Fallujah were fierce, with a barrage of rocket, mortar and gunfire raining down as they tried to raise the new Iraqi flag above the hospital.

The Pentagon said US forces seized the hospital first to provide medical care but also in the expectation that the presence of embedded reporters at the hospital would prevent inflated reporting of civilian casualties.

Allawi said that 38 insurgents had been killed in the initial clashes and four foreign fighters detained, including two Moroccans.

The battle could prove the most intense since last year's war to topple Saddam Hussein, with 2,000 to 2,500 fighters, some loyal to Iraq's most wanted man Abu Musab al-Zarqawi, inside the city and prepared for brutal street fighting.

"The predictions are that they are going to stay and fight us here," said Major Todd Desgrosseilliers. A combined force of some 10,000 US and 2,000 Iraqi troops are involved in the offensive.

The action came one day after Allawi declared a 60-day state of emergency across most of the country in a bid to control an escalating insurgency ahead of elections promised by January.

US troops forbade men aged from 15 to 50 from entering or leaving Fallujah and the surrounding areas, saying that anyone else who wanted to leave would not be allowed to return until order is restored.

"Attention, attention! All men aged between 15 and 50 are forbidden from entering or exiting (the area)," loudspeakers on top of US vehicles declared in Arabic as they drove around the outskirts of the city.

"Only women and children are allowed to leave on condition that they do not return until order is restored."

Rebels have transformed Fallujah into their fiefdom since a marine assault in April ended in stalemate and left hundreds dead. It is estimated now that 80 to 90 percent of the city's 300,000 inhabitants have fled.

US and Iraqi troops have been massing around the city since mid-October, while the US military is doubling its manpower in Fallujah's sister city of Ramadi to 2,000 amid expectations of a double-pronged assault to regain control.

As part of the security measures, Allawi said Iraq's international airport would be closed to civilian flights for 48 hours and the country's borders with both Jordan and Syria would be closed except for trucks carrying necessary food.

Allawi also declared a curfew from 6:00 pm (1500 GMT) would be imposed on the restive cities of Fallujah and neighbouring Ramadi along with other emergency measures.

With instability still rife elsewhere in the country, a US soldier was killed when gunmen fired on a military patrol in eastern Baghdad, the US military said.

And at least three people were killed and 45 wounded when two suspected car bombs exploded within minutes of each other outside two Christian churches in southern Baghad.

Meanwhile, at least eight Iraqis were killed and more than a dozen people wounded, including an American soldier, in attacks Monday in central and northern Iraq, officials said.

In the restive Sunni city of Ramadi, at least four Iraqis were killed and one wounded in a car bomb attack as a US convoy was passing, said police.

Near the city of Samarra north of Baghdad, two Iraqi contractors working with the Americans were killed and two others were wounded when gunmen opened fire on their vehicle, a police spokesman said.

US and Iraqi forces, meanwhile, searched for suspects for a second day in Samarra amid reports of intermittent clashes there.

Police said Iraqi national guardsmen and US troops conducted sweeps in some parts of Samarra, two days after car bombs and attacks on police stations in the city left 36 dead and scores wounded.

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