The claims of both sides are impossible to verify
At least 378 people have been killed by fierce shelling from the Sri Lankan army in the past 24 hours, a health official has told the BBC.
The doctor, 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.
The army denies shelling the designated "safe zone" for civilians, and blames any deaths on Tamil Tiger rebels.
The claims are impossible to verify as reporters are banned from the war zone.
But the BBC's Charles Haviland, in Colombo, says a steady stream of information coming from the area suggests that civilians are being killed.
And he says health officials are convinced that the shells are coming from territory held by the Sri Lankan army.
The pro-rebel Tamilnet website reported that the army began to fire artillery shells late on Saturday.
The site said as many as 2,000 civilians had been killed.
Dr V Shanmugarajah said he could not confirm that figure but said the makeshift hospital he is working in - at a school in east Mullaivaikal in Mullaitivu district - had so far taken in 378 bodies.
He said 106 of those killed were children.
Sri Lanka has deported three British television journalists arrested on charges of tarnishing the image of the Sri Lankan security forces.
The three had compiled a report for the London-based Channel Four news, about allegations of poor treatment and sexual abuse at camps for those fleeing the fighting between Sri Lankan forces and Tamil Tiger rebels.
The journalists were detained in the eastern city of Trincomalee on Saturday.
Lakshman Hulugalle, who heads the government security information centre, said the trio had admitted they had "done something wrong".
A spokeswoman for the British Foreign Office said Sri Lanka's decision to deport them was disappointing when the case for greater transparency in Sri Lanka was overwhelming.
Claims of duress
In response to claims of civilian deaths, military spokesman Brigadier Udaya Nanayakkara said radar had detected Tamil Tigers themselves using artillery and mortar fire on two occasions on Saturday morning, directed against civilians within their zone.
"These doctors are giving statements based on some of the false propaganda given by the LTTE [Tamil Tigers]," he said.
"Maybe there is an LTTE gun pointing at them and asking them to give a statement. All these stories are exaggerated to tarnish the image of the Sri Lankan troops who are fighting the LTTE terrorists."
Sri Lankan defence spokesman Keheliya Rambukwella also told the BBC that reports of government shelling were "propaganda" of the Tigers.
He said the guerrillas were "holding people to ransom" in their area, and accused them of killing nine civilians who were trying to escape their zone on Saturday.
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 a government nursing officer was among those killed.
The UN estimates that about 50,000 civilians are trapped by the conflict in a three-km-sq strip of land.
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.