"Sometimes it's more difficult than orginally planned," said marine captain Paul Batty.

"What's happening now is to the credit of these (rebels) and their will to fight. They are here to die.

"Right now we are fighting fanatics -- it's way beyond the money question. You would need millions of dollars to get an ordinary person to live this life."

After days of relative calm, fighting erupted Friday in the former rebel stronghold and continued through the weekend in some of the heaviest clashes since last month's US-led assault to wrest the city from insurgents, an AFP correspondent embedded with the marines said.

Some 11 US troops have been killed in fighting since Saturday in Al-Anbar province, which is home to Fallujah, though the US military would not specify if the deaths occurred in the city.

Marines involved in the fighting say as many as 200 rebels could still be hiding in ruined neighborhoods on Fallujah's north side.

At least 30 suspected insurgents were killed in fighting over the weekend, Batty said, but he added that marines also took heavy casualties with more than a dozen wounded.

"We had to call in tanks and air support to finish them off," Batty said.

Despite the massive amount of firepower unleashed on Fallujah during last month's assault which began November 8 and lasted about a week, insurgents in the city have proven wily opponents as marines try to police up the city in the aftermath of the initial attack.

"This is a smart enemy, even though their fighting technique is average with little discipline. They are incredibly skilled with IEDs (improvised bombs) and booby traps," Batty said.

He said at least 60 percent of the fighters his units has encountered were foreigners.

"There are people from Egypt, Syria, Saudi Arabia, we even had Chechens," he said.

Batty said that even though marines have sealed the city, "if you're on foot you could get it. It's possible some have managed to get in".

Tens of thousands of Fallujah residents who fled the city before last month's assault are expected to begin returning in the coming weeks and marines fear insurgents will be hiding among them.

"It will be very difficult," Batty said, especially since large swathes of the city -- including businesses and homes -- have been devastated by the fighting.

"We think people will be very upset when they see the level of damage inflicted to their town. I know I would be," Batty said.

He said each family would receive 500 US dollars from the marines

"Some will blame the coalition forces, others will blame the mujahadeen," he said. "Most will take it as a result of fate ... I guess it's the price of freedom."