The attack, inside an official "no-fire zone", was among the bloodiest incidents since the army began its offensive against Tamil Tiger rebels.

Photo from pro-Tamil organisation Mercy Mission to Vanni shows injured civilians at a makeshift hospital in Sri Lanka's northeastern district of Mullaittivu. Sri Lanka's Tamil Tiger rebels have accused government forces of killing at least 47 people in an artillery and mortar attack on a hospital, a charge fiercely denied by the island's military. (AFP/HO) About 50,000 civilians are now trapped inside a tiny strip of coastline, covering an area less than twice the size of London's Hyde Park. President Mahinda Rajapaksa's government has repeatedly promised not to use heavy artillery in this zone.

But repeated and deadly bombardments have taken place - although possibly with mortar bombs rather than artillery. The rudimentary hospital in Mullivaikal, consisting of little more than blue tarpaulins draped beneath a tin roof, was hit by one bomb, with the crowded admissions area taking the worst of the blast.

Photographs taken after the explosion showed bodies strewn in the dirt and medical personnel struggling to help the wounded. Dr Thurairaja Varatharajah, the senior government health official in the war zone, said that about 50 people were wounded in the attack and the death toll could rise.

The government initially denied any knowledge of the incident and then said the army had not been responsible. The "no-fire zone" is still under rebel control and the authorities have accused the Tamil Tigers of holding human shields, inventing attacks or even staging their own atrocities in order to blame the government.

But outside observers believe the army has been responsible for many of the worst incidents. The United Nations has described the shelling of the "no-fire zone" as a "bloodbath" and a foreign doctor estimated that at least 430 people, including 106 children, died last weekend alone.

Ban Ki-Moon, the UN secretary-general, said he was "appalled" by the carnage, adding: "Thousands of Sri Lankans have already died in the past several months due to the conflict, and more still remain in grave danger."

Mr Ban urged both sides to refrain from using heavy weapons and singled out the Tamil Tigers, accusing them of "reckless disrespect" for the safety of civilians by allegedly holding them hostage for use as human shields.

The hospital at Mullivaikal had been shelled at least three times before. Human Rights Watch (HRW) has documented 30 cases of medical centres being shelled in the last five months. Deliberately attacking a clearly marked hospital or clinic is a breach of humanitarian law.

HRW investigators have concluded that the army was responsible for many attacks. "The Sri Lankan armed forces have repeatedly struck hospitals in the northern Vanni region in indiscriminate artillery and aerial attacks," said the organisation. "Commanders responsible for ordering or conducting such attacks may be prosecuted for war crimes."