The week-long battle for Fallujah has destroyed Iraq's most notorious rebel hideout, but militants have taken the fight elsewhere, opening up new fronts in Mosul, Ramadi and Baquba, where more than 20 people have been reported killed.

In an audiotape posted on an Internet site, a man claiming to be Abu Musab al-Zarqawi, Iraq's most wanted man, exhorted insurgents around the country to rise up against US-led forces and brace for new battles.

The Fallujah assault has also raised fears about a humanitarian disaster with tens of thousands of residents forced to flee the Sunni Muslim city, while the plight of those who stayed behind remains uncertain.

American troops prevented a Red Crescent aid convoy from entering Fallujah after two days of negotiations, because they said it was too dangerous.

In Baquba, 60 kilometres (37 miles) northeast of Baghdad, US-led forces battled with up to 150 insurgents, killing at least 21 of them, said William Chlebowski, the acting US commander.

"It was significant. It was timed to happen at the end of Ramadan during a time when many Iraqi security forces were not at work, it was their holiday," he said.

Another senior officer, Major Art Weeks, said 19 insurgents were killed when US aircraft dropped two 500-pound (225-kilogram) bombs as they tried to move heavy mortars around in two trucks.

An Iraqi police officer put the number of dead at seven rebels and three policemen, including a local police chief.

The casualty figures could not be independently verified.

While Baquba has a roughly even Sunni and Shiite population, much of the violence has flared in the predominantly Sunni cities of Mosul, Ramadi, Samarra and Baiji.

Military commanders had warned that the battle for Fallujah would fan unrest across the Sunni Muslim heartlands of central and northern Iraq.

"Is this linked to Fallujah, or to take some of the pressure off of Fallujah? It's too early to say but possibly," said Chlebowski.

Iraq's northern city of Mosul was calmer on Monday.

But Iraq's interim Interior Minister Falah al-Nakib warned of worse to come, after describing the gruesome murder of a policemen during clashes in Mosul the previous day that left seven policemen and more than 30 insurgents dead.

"They kidnapped a wounded policeman from hospital and cut him into pieces while he was injured, then he was hung in an area and then policeman came and cut him down," Nakib told a televised news conference.

"Today it is quieter in Mosul but we expect a surge in attacks in the coming two days," the minister said, adding that the government would be ready.

US and Iraqi troops, meanwhile, pushed on with a clean-up operation in Fallujah, west of Baghdad, after warplanes blasted the southern area of the city Monday morning in the hunt for the last pockets of resistance.

The city is effectively under US and Iraqi control, but the military has yet to declare the end of the largest operation since last year's invasion of Iraq, as it rakes through the devastated streets.

Victory in Fallujah will come at a price as the death toll on both sides mounts and tens of thousands of civilians remain in makeshift camps outside.

At least 39 US soldiers have been killed and 275 wounded so-far, while at least five Iraqi troops and more than 1,200 insurgents have also died, according to the US military.

During their painstaking, house-to-house search, marines found the bodies of 12 Arab men who had apparently been shot dead execution-style, said an AFP reporter, who has seen in a week 27 corpses that were probably executed.

Meanwhile, in the tape posted on an Islamist website, the man claiming to be Zarqawi urged the rebels to rise up.

"The enemy ... has mobilised most of its assets and potential to destroy Islam in Fallujah. Once they have finished in Fallujah, they will head towards you," he said.

"Be cautious and foil their plan," he said, in an implicit recognition that the battle in Fallujah was virtually lost for the insurgents under US assault.

Iraqi Prime Minister Iyad Allawi, meanwhile, said an Islamist rebel group linked to Zarqawi had been crushed and its leaders arrested during the offensive.

"We have our hands on the group known as Jaish Mohamed (Mohamed's Army), its chief Muayyed Ahmed Yassin, alias Abu Ahmed. We have also arrested his lieutenants and the group's command," he told Al-Arabiya television.

Fallujah was home to some 300,000 people, two-thirds of whom were estimated to have fled before the offensive began. But there are no accurate figures for those remaining, lacking food, water, electricity and medical facilities.

The US military refused permission for a Red Crescent convoy carrying food, water and medical supplies to advance, arguing its troops could do the job, that only a few civilians are involved and it was too dangerous for the convoy.

Hitting Iraq where it hurts the most economically, saboteurs blew up a section of an oil pipeline in the northern region of Kirkuk, while flames raged in four oil wells after a string of bombings the previous day, officials said.

In simmering unrest, six Iraqis, including four children, died when mortar rounds hit a southern district of Baghdad, witnesses and a doctor said.

The international airport in the capital, where US Joint Chiefs of Staff chairman General Richard Myers on a surprise visit met Allawi on Monday, reopened to civilian traffic after being shut for a week.

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