US Marines met ferocious resistance in the western Iraqi town of Fallujah today as they pressed a four-day offensive against Sunni Muslim insurgents, prompting their commander to make comparisons with the Vietnam war.

As the Marines inched forward block-by-block taking sniper fire and hit-and-run attacks with mortars and rocket-propelled grenades, a US medic said the resistance was more intense than in last spring’s invasion.

Mortar and small-arms fire were launched by small groups of insurgents who materialised from alleyways or on rooftops, only to melt away again.

The thud of mortar rounds echoed around the town and plumes of smoke dotted the landscape. Machine-gun fire rattled through the streets as F-16 warplanes buzzed overhead for surveillance.

After more than three days of ferocious fighting, the Marines had managed to move just a little over a mile through an industrial zone, on the eastern edge of the town, which they had thought was largely abandoned.

By this afternoon, the Marines had stopped their advance to wait for reinforcement from a third battalion, officers said.

"Right now we’re just holding fast," said First Lieutenant Luke Pernotto, of the 1st Battalion, 5th Marines."We tried to move a little bit, but we don’t have enough people to push the insurgents back."

"Reinforcements will help us a lot. They shoot at us and we don’t have enough people to go seal off where they’re going back to."

The flames of exploding rockets had lit the sky as the Marines came under repeated mortar and RPG fire from factories, homes and mosques, some of it from areas supposedly already cleared.

"MOUT (Military Operations in Urban Terrain) is the most intense kind of fighting," said Lieutenant Colonel Brennan Byrne, a battalion commander.

"And this is like Hue City in Vietnam," he said referring to the former imperial capital where in 1968 US troops faced the most ferocious street fighting of the communists’ decisive Tet offensive.

Marines, who had taken part in the defeat of Saddam Hussein’s armed forces a year ago, said the resistance they were facing from the insurgents in Fallujah was tougher than anything thrown at them by the old regime’s once feared Republican Guards.

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