At least 378 people have been killed by continuing shelling from the Sri Lankan army in the past 24 hours, a health official has told the BBC.
The official, working in the northern conflict zone, said 1,122 others had been injured – and more bodies were on beaches and by the sides of roads.
But the Sri Lankan military denies it has shelled the "safety zone".
Tamil Tiger rebels and the military regularly accuse each other of atrocities in the civil conflict.
But the claims are impossible to verify because independent reporters are banned from the war zone.
The pro-rebel Tamilnet website reported that heavy shelling had started late on Saturday and continued into Sunday.
The site said as many as 2,000 civilians had been killed, but health officials said they could not confirm the figure.
The BBC’s Charles Haviland, in Colombo, said an official in the war zone confirmed his hospital had taken in 378 bodies so far.
‘Propaganda’ claims
Earlier, doctors said two hospitals were struggling to cope with the casualties, and that people were hiding in bunkers and many makeshift tents had been burnt.
They added that among those killed was a government nursing officer.
But the military denied the allegations.
Sri Lankan defence spokesman Keheliya Rambukwella told the BBC the accounts were "propaganda" of the Tamil Tigers and that there had been neither shelling nor air attacks on the civilian zone.
"That is a fact," he said.
He said the guerrillas were "holding people to ransom" in their area, and accused the rebels of killing nine civilians who were trying to escape their zone on Saturday.
The UN estimates that about 50,000 civilians are trapped by the conflict.
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