COLOMBO - DOCTORS and patients fled a hospital in northern Sri Lanka - closing the last medical facility in the war zone - after days of artillery attacks by government and rebel forces, aid groups said.

With concern growing for the estimated 250,000 ethnic Tamils trapped in the conflict zone, the UN said cluster munitions appeared to have hit near the hospital on Wednesday and that 52 civilians were killed elsewhere the previous day.

As fighting raged, President Mahinda Rajapaksa presided over an Independence Day celebration and reasserted that the government was on the brink of destroying the Tamil Tiger rebels and ending the 25-year-old civil war.

'I am confident that in a few days we will decisively defeat the terrorist force that many repeatedly kept saying was invincible,' he said.

The US, Britain and Canada urged both sides to agree to a temporary cease-fire to allow civilians and the wounded to leave the conflict zone and asked that humanitarian agencies be given access.

Sri Lanka barred nearly all aid groups from the war zone last year.

With the military pounding the area with artillery and airstrikes to finish off the rebels - and the rebels hitting back with their own artillery - there have been more reports of heavy civilian casualties.

UN spokesman Gordon Weiss said 52 civilians were killed and 80 wounded on Tuesday in and around a government-designated 'safe zone,' an area of rebel territory that the government had pledged not to strike.

After days of artillery fire killed at least 12 people at the Puthukkudiyiruppu hospital, patients began fleeing. On Wednesday, the Red Cross evacuated the staff and remaining 300 patients.

'The last remaining medical facility inside the Vanni pocket (in the war zone) has been effectively closed,' Mr Weiss said.

The doctors and patients went to a coastal area deeper inside the war zone where there was no reliable source of drinking water, said Sarasi Wijeratne, spokeswoman for the International Committee of the Red Cross.

Aid workers were trying to find them a better place to stay or get the two sides to grant them safe passage out of the conflict zone, she said.

Mr Weiss said 15 UN staffers and 81 family members who were trapped near the hospital also fled after the area was pounded for more than 16 hours by artillery fire, including what appeared to be cluster munitions.

Dr. Thurairajah Varatharajah, the top health official in the war zone, estimated last week that more than 300 civilians had been killed in recent fighting. Varatharajah has not updated his estimate. The government denies any civilians have been killed.

About 12,000 people in Geneva and 4,000 in Berlin protested against Sri Lanka's offensive on Wednesday. -- AP