UPDATE: Fresh Heavy Fighting In Northern Sri Lanka

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(Adds military quote, details of fighting)

COLOMBO (AFP)--Sri Lanka's Tamil Tigers on Wednesday launched a wave of suicide attacks against advancing government soldiers, the military said, as heavy fighting raged despite renewed international calls for a truce.

The clashes came as both sides in the brutal conflict faced renewed charges of war crimes, with the U.N. describing the situation as "absolutely awful" and a rights group saying civilians were being used as "cannon fodder."

U.S. Secretary of State Hillary Clinton and her U.K. counterpart David Miliband also repeated so far futile demands that the fighting stop and thousands of trapped civilians be allowed to escape.

But the island's military, which says it has the Liberation Tigers of Tamil Eelam, or LTTE, surrounded in just four square kilometers of northeastern coastal jungle, signaled both sides were determined to battle to the end.

Military spokesman Brig. Udaya Nanayakkara said LTTE fighters mounted a ferocious counterattack, with at least 10 guerrillas dying in a wave of suicide boat strikes along the coast.

"We have successfully repulsed their counterattacks and search operations are now under way," Nanayakkara said. Several government soldiers were reported wounded, but no further details were given.

There was no immediate comment from the rebels, and the army's claims can't be independently verified as independent journalists, diplomats and most aid agencies are barred from going near the conflict zone.

The Colombo government estimates up to 20,000 civilians are being held in the pocket where the LTTE are holed up, although the U.N. has said as many as 50,000 might be trapped - huddled in shallow bunkers and with scant food, water or medical facilities.

On Tuesday, the rebels accused the army of killing at least 47 civilians in an artillery and mortar barrage that struck a hospital, a charge fiercely denied by the island's military.

That followed an alleged massive army bombardment of the area over the weekend, which the rebels claimed had left up to 2,000 dead.

Human Rights Watch blamed both sides for the ongoing carnage.

"Recent satellite photos and witness accounts show the brutal shelling of civilians in the conflict area goes on," said Brad Adams, Asia director at the U.S.-based rights watchdog.

The group also described witness accounts of how the Tamil Tigers - who have been fighting for a separate state since the 1970s and once controlled a third of the island - murdered anyone who tried to cross into government territory.

"Neither the Sri Lankan army nor the Tamil Tigers appear to have any reluctance in using civilians as cannon fodder," Adams said.

U.N. humanitarian co-ordinator John Holmes said intransigence by both the Sri Lankan government and the rebels had created an "absolutely awful situation."

"The LTTE are clearly still holding onto that population against their will, using them as human shields," he said in Geneva.

"The government has said they aren't using heavy weapons. But the evidence suggests that they are continuing to do so, at least to some extent."

The U.N.'s human rights chief Navi Pillay has already said both sides might be guilty of war crimes.

On the diplomatic front, the U.K and U.S. issued a joint appeal for a truce - the latest in a series of so far futile international calls for the bloodshed to stop.

"Secretary Clinton and Foreign Secretary Miliband call on all sides to end hostilities immediately and allow for the safe evacuation of the tens of thousands of civilians," a statement said after the two met in Washington.

The European Union has called for the issue to taken up by the U.N. Security Council, but some powerful members, notably China and Russia, are opposed.