Seven air strikes leveled arms caches stored in the Sunni Muslim stronghold over a 24-hour span, the US marines said, as the military intensified its campaign to wipe out insurgent positions ahead of an anticipated invasion.

Four Iraqi civilians were wounded, said Doctor Ali Hamis at the Fallujah general hospital.

The marines relentlessly pounded the insurgent enclave with artillery fire from Saturday night through Sunday morning, an AFP correspondent embedded with marines reported, as residents of Fallujah continue to flee.

Iraqi Prime Minister Iyad Allawi has issued an ultimatum to the city to surrender its rebel fighters, believed to include those loyal to Al-Qaeda's frontman in Iraq Abu Musab al-Zarqawi, or face attack.

At least 12,000 troops, including four US military brigades and one Iraqi forces brigade -- are ringing the city, a marine officer told AFP.

They are awaiting a final go-ahead from Allawi, who is desperate to crush pockets of resistance in Iraq ahead of national elections promised by January.

The prime minister is due to address reporters Sunday.

In a worrying challenge to the effectiveness of using military muscle to restore order, a torrent of car bombs and clashes left at least 33 people dead and 48 wounded in the Sunni Muslim stronghold of Samarra on Saturday.

US and Iraqi troops stormed the city north of Baghdad at the start of October to wrest it from rebel hands in what was at the time heralded as a successful campaign.

Out-gunned but still ready to fight, gunmen stormed two police stations in the restive province of Al-Anbar, where Fallujah is situated, at dawn on Sunday, shooting dead, execution-style, 21 policemen.

Iraq's US-trained security forces are prime targets in the insurgency, as they are much easier to hit than the better-equipped US military.

In final preparations for what might be their hardest fight since last year's US-led invasion, marines expressed last-minute jitters.

A gunnery sergeant, doing a last minute check on supplies, said fighting was much different for him now than when he headed off to Somalia in 1992.

"I have a family now. I'm scared," he confessed.

Rebels have transformed Fallujah into their fiefdom since a marine assault on the city in April ended in stalemate and left hundreds dead. Tens of thousands have fled the 250,000-strong city in fear of greater bloodshed.

A US marines commander, Lieutenant Colonel Patrick Malay, predicted a brutal conflict if his troops poured into the city.

"These are some of the most cowardly vicious opponents we've ever faced," the Malay told a crowd of marines on Saturday.

"They hide among civilians, among women and children. They fire from mosques and schools. They use ambulances to move. Gentlemen, it's going to be nasty in there," he warned.

In case of battle, Malay said the marines expected to face car bombs, snipers, suicide bombers, booby-trapped buildings -- meant to collapse on troops, roadside bombs, landmines and devices like mobile phones packed with plastic explosives.

Many side streets off the main east-west road cutting through Fallujah had been barricaded with cement blocks and sandbags to guard the fighters' movements against US snipers, an AFP correspondent inside the city reported.

The fighters had also set up various positions in deserted and destroyed parts of the rubble-strewn city, but they only show up there when US war planes strike or shelling starts, the correspondent said.

Each fighter belongs to a squad that is itself part of a higher unit. The leaders keep a low profile and are only known to their men, the correspondent said. A marine officer gave a near identical account.

US commanders estimate there are 2,000 to 2,500 fighters in the city and its surrounding areas, which they describe as the insurgency's nerve centre, along with another 10,000 men who could join the fight.