COLOMBO: Sri Lanka's Tamil Tiger rebels have accused government forces of killing at least 47 civilians in an artillery and mortar attack on a hospital, a charge fiercely denied by the island's military.

A spokesman for the LTTE, whose fighters are battling to defend a tiny strip of coastal jungle, said on Tuesday that a makeshift hospital packed with survivors of weekend attacks had been hit by government forces.

"Heavy artillery and mortar attacks this morning hit the hospital and 45 people were killed there," rebel spokesman S. Puleedevan said by telephone. "Most of them were those injured in Sunday's attacks."

News of the claimed hospital attack comes as US Secretary of State Hillary Clinton, and her British counterpart David Miliband, called on Sri Lankans to stop fighting immediately and allow trapped civilians to escape the conflict.

Their joint appeal on Tuesday was the latest in a series of so far futile international calls aimed at ending the fighting between government forces and Tamil Tiger separatist guerrillas, holed up on a coastal strip in the island's northeast.

Refering to the alleged hospital attack, the pro-rebel Tamilnet website said the number killed in the bombardment had since risen to 47, and that 55 patients had been wounded. The LTTE says more than 2,000 civilians were killed in similar alleged Sri Lankan army attacks on Sunday.

The government's main moderate Tamil political ally also confirmed the civilian losses, but stopped short of apportioning blame and asked the government to intervene and urgently help the victims.

"Please intervene and stop firing shells targeting terrified civilians," the leader of the moderate Tamil United Liberation Front (TULF), V. Anandasangari, said in a letter to President Mahinda Rajapakse.

Anandasangari asked the government to send a medical team to Mullivaikal where over one thousand casualties of other incidents were also awaiting treatment for two days.

"Even if this is an act of the LTTE (Liberation Tigers of Tamil Eelam) the government has a duty to save the civilians,' he added.

The remarks came amid mounting international outrage over large-scale civilian deaths, with the European Union calling for UN Security Council action and the US telling Sri Lanka to stop using heavy weapons.

Sri Lanka's defence ministry however blamed any civilian deaths on the rebels -- who it accused of firing heavy weapons from inside an area declared to be a "civilian safety zone."

The military insists it is not using heavy guns, and says the Tigers are keeping people hostage to use them as human shields and shooting those who try to escape.

The rival claims cannot be independently verified as independent journalists, diplomats and most aid agencies are barred from going anywhere near the conflict zone in the far northeast of the island.

The International Committee of the Red Cross (ICRC) did confirm there was firing in the area, and that it had been unable to evacuate the wounded from the make-shift Mullivaikal hospital. It did not comment on who was responsible.

It said it could not operate a mercy mission to ferry out the wounded from the small sliver of coastal land still in the hands of the Tamil Tigers on Tuesday because of continuing fighting.

"The plight of the people remaining in the combat area is desperate," said Paul Castella, the head of the ICRC delegation in Sri Lanka. "We need unimpeded access to them in order to save lives."

Sri Lanka says the Tigers, who have been fighting for an independent Tamil homeland since the 1970s, now control only four square kilometres (1.5 square miles) of coastal jungle and are on the brink of total defeat.

The defence ministry said its troops had captured more ground in the latest fighting and had recovered 35 rebel bodies. The military spokesman said the LTTE's leader, 54-year-old Velupillai Prabhakaran, was now trapped.

"According to our information, the LTTE leadership, along with their second-rung, is remaining in the small area," Nanayakkara said.

On Monday, UN Secretary-General Ban Ki-moon called for an immediate halt to the war -- saying he was "appalled at the killing of hundreds of civilians in Sri Lanka over the weekend".

The UN spokesman in Colombo has described the weekend's events as a "bloodbath".

Despite the outrage, the Sri Lanka issue is not on the agenda of the 15-member council as some powerful members, notably China and Russia, are opposed to taking action.