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COLOMBO, Sri Lanka - Artillery shells tore through a hospital packed with wounded civilians in Sri Lanka's war zone for a second day Wednesday, killing at least 50 people, setting an ambulance ablaze and forcing the medical staff to huddle in bunkers for safety, doctors said.

Health workers at the makeshift medical facility said they were so overwhelmed by the crush of the wounded and the unrelenting shelling of the area they could do little but give gauze and bandages to the roughly 1,000 patients waiting for treatment.

The strike on the hospital came as the government marched on with its offensive to destroy the reeling Tamil Tiger rebels and end their quarter-decade quest for a separate homeland.

There has been a wave of artillery bombardments across the war zone that began over the weekend and has barely let up in five days, health workers said. The weekend attacks alone may have killed as many as 1,000 people, doctors said.

The government says its troops are not responsible for the shelling and that the military has not fired heavy weapons in the area in weeks.

But Human Rights Watch says satellite images and witness testimony contradict that claim and has accused both sides of using the estimated 50,000 civilians packed into the tiny coastal strip controlled by the rebels as "cannon fodder."

The shelling was so intense Wednesday that a Red Cross ferry waiting off the coast to deliver food aid and evacuate the wounded had to turn back for a second day, the agency said.

U.S. President Barack Obama on Wednesday called for an end to the violence and steps to alleviate civilian suffering.

Obama called on Sri Lanka's government Wednesday to "stop the indiscriminate shelling" that has killed hundreds of civilians and allow UN humanitarian teams access to war-wounded.

He also called on the Tamil rebels to lay down their weapons and release civilian captives. He said the situation was a humanitarian crisis that could turn into a catastrophe.

Two artillery shells hit the medical facility about 1 p.m., slamming into an administrative office and a ward filled with patients already wounded by previous shell attacks, Dr. V. Shanmugarajah told The Associated Press by telephone.

Dr. Thurairaja Varatharajah, the top health official in the war zone, said the attack killed at least 50 people, including patients, relatives and a health aide, and wounded about 60 others.

Heavy shelling of the war zone continued throughout the day, he said.

"We are unable to treat the people properly because a lot of aides have fled the hospital. We go into bunkers when there is shelling and try to treat them as much as we can when there is a lull," he said by telephone.

One shell that struck the compound set an ambulance on fire, according to video footage of the aftermath of the attack. Scores of people crowded beneath tarps outside the hospital building waited for care as someone wailed in the background.

More than 1,000 civilians -- many with amputations or chest wounds -- had been waiting for treatment at the hospital when it was struck, and every 10 minutes or so another one or two died from lack of care, said a third hospital official, who spoke only on condition of anonymity because the government had not authorized him to speak to the media.

Overwhelmed doctors have been reduced to handing out bandages to the seriously wounded, the official said. More than 100 dead bodies have been left inside the compound because no one will risk burying them amid the constant shelling, he said. The strike was the third to hit the hospital this month.

A Red Cross worker and his mother were also killed in shelling, the third Red Cross staff member killed by shrapnel in the war zone in the past two months, the International Committee of the Red Cross said.

"This latest tragic incident shows how dangerous it is for everyone in the area," said Paul Castella, the head of the Red Cross delegation in Colombo.

Rebel spokesman Seevaratnam Puleedevan said shells also hit a home for mentally handicapped women, killing 38 and wounding more than 40.

Reports of the fighting are difficult to verify because the government has barred journalists and aid workers from the war zone.

The government has come under heavy international criticism for the large civilian toll of its offensive against the rebels, who are cornered in a five square-kilometre strip of land.

The military said it pressed ahead with its offensive into that strip Wednesday, capturing one of the rebels' heavy guns and fending off a suicide attack by four rebel boats laden with explosives.

The UN Security Council demanded Wednesday that the Tamil Tigers stop fighting and allow tens of thousands of civilians being used as human shields to leave the war zone. It also called on the government to stop firing heavy weapons, help trapped civilians evacuate and allow for the urgent delivery of humanitarian aid.

The council press statement -- which is not legally binding -- expressed grave concern at the worsening humanitarian crisis.

Diplomats in Geneva said the United Nations was preparing for an emergency meeting of its Human Rights Council next week to discuss the worsening conflict. Fifteen of the 16 countries needed to back the motion were already on board, European diplomats said, speaking on condition of anonymity because of the sensitivity of the issue.

Amnesty International urged Obama to push for a truce and appealed to the UN Security Council to establish a commission of inquiry into violations of international law.