The rebels say civilians have nowhere left to go
Sri Lankan government forces have shelled a hospital inside the conflict zone killing at least 49 people, Tamil Tiger rebel and hospital sources say.
The rebels said the makeshift hospital in Mullivaikal was hit in the morning.
The Sri Lankan government denied the army had caused civilian casualties or used heavy weapons in civilian areas, but said it had pierced rebel defences.
As it advanced south, it also said that all voices speaking from the Tiger-held zone amounted to misinformation.
Speaking in Geneva, the UN's top aid official, John Holmes, accused the Tamil Tigers of holding civilians against their will, adding that there was evidence that the government was continuing some use of heavy weapons despite its denials.
The claims are impossible to verify as reporters are banned from the area.
More than 400 people were killed and over 1,000 injured over the weekend in what the UN has described as a bloodbath.
Many of those sick or injured were supposed to have been evacuated by the International Committee of the Red Cross (ICRC) on Tuesday
It said that it was unable to collect them because of continued heavy fighting.
The rebels say that the medical centre in Mullivaikal does not have enough food or medicine to treat scores of injured people.
Adam Mynott , World Affairs correspondent
As the human suffering has increased so has the clamour of the international community. But the growing intensity of worldwide concern and demands for the killing to end has, so far, been powerless to stop or arrest the fighting.
Delegations of heavy-weight international politicians have beaten a trail to Colombo. During the past month UN humanitarian chief John Holmes left after a three-day visit without being able to persuade Sri Lanka to open a humanitarian corridor to the rebel-held territory. The British and French foreign ministers, David Milliband and Bernard Kouchner, led another delegation but apparently had little or no impact.
The UN Security Council has discussed the deteriorating situation but Sri Lanka has not been a full agenda item in a Security Council meeting because of a reluctance by Russia and China to include it. They view the issue as an internal matter, and this has undoubtedly emboldened the government in Colombo to remain unmoved by international pressure.
Meanwhile, some foreign aid agencies say they are becoming increasingly concerned over the safety of their staff in the war zone.
One such agency contacted by the BBC said that they were "very worried" about three Tamil staff who they had not been in contact with for three days.
A spokesman, for the agency which did not want to be named, said that up until now the trio had been in daily contact.
In their last message they said that food shortages were acute and that the fighting had taken a turn for the worse.
The ICRC's spokeswoman in Colombo, Sarasi Wijeratne, told the BBC that a ship which had been waiting off the rebel-held zone to deliver essential supplies and evacuate sick and wounded people was returning to the eastern town of Trincomalee.
"The prevailing ground situation has prevented aid workers on board a relief vessel to off-load supplies and to evacuate sick and injured people from the conflict zone," she said.
"The vessel is expected to return to Mullivaikal area tomorrow."
The last evacuation was on Saturday. Ms Wijeratne said supplies had been getting through but they were not enough to meet people's basic needs.
Thurairaja Varatharajah, a health official at the hospital, told the BBC Sinhala service it had been hit by a single mortar shell.
More than 80 people were injured, and he expected the death toll to rise because many of the injured had bad head and stomach wounds, he added.
However, military sources have suggested that doctors in the zone may be being forced to give statements at gunpoint by rebels.
Rebel spokesman Seevaratnam Puleedevan told the AP news agency that civilians were fleeing in all directions.
"There's no place to seek shelter or protect themselves," he said.
The Sri Lankan military has meanwhile blamed civilian deaths on the Tigers, saying they are using people as human shields.
Defence ministry spokesman Keheliya Rambukwella told the BBC he knew nothing of the incident but that the military was not deploying heavy weaponry in civilian areas.
The UN estimates that about 50,000 civilians are trapped by the conflict, in a three-sq-km strip of land. Most of this area has been designated by the government as a safe zone which will not be attacked by air or by heavy weapons.
UN Secretary General Ban Ki-moon said on Monday he was appalled by the killings and called for an immediate halt to the fighting.
Diplomats from the UK, France and Austria said the Security Council should address the crisis, while the US said it was "deeply concerned" by the crisis.
The Tamil Tigers have fought for an independent homeland for Sri Lanka's Tamil minority since 1983.
More than 70,000 people have been killed in the war.
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