Its top humanitarian official, John Holmes, who visited holding camps in the north of the island earlier this week, said he had received reliable reports of continued assault despite government assurances that it would stop.
His comments were backed by Western diplomatic sources in Colombo who confirmed reports of continuing shelling in the no-fire zone where an estimated 50,000 civilians are trapped.
Human rights campaigners have now called for a ‘war crimes’ investigation into allegations of abuses by both sides in the conflict.
Claims that civilians had been killed were backed up by a government doctor who said up to six patients had been killed in mortar attack on a hospital inside the zone.
Dr. Thangamuttu Sathyamurthi said a dozen shells struck the hospital where he was working. "I saw four dead bodies on the side of the road outside the hospital and two bodies in a trench," he said.
Army spokesman, Brigadier Udaya Nanayakkara, denied the area had been shelled or attacked with heavy calibre weapons, though he confirmed fighting continued inside the zone. He said government forces had captured a Tamil Tigers defensive structure, killing seven fighters and recovering a cache of weapons.
He said a raid by special forces commandos at 2am had taken an LTTE "bund", a 12ft high and 760 yard long mound which had stopped civilians escaping.
"We have not used any shelling or mortaring, only small arms. We know how we are fighting," he told The Daily Telegraph.
The UN claims highlight distrust of the government’s pledge not to use heavy weapons and appear to corroborate earlier LTTE allegations that 139 civilians were wounded in shelling after the presidential announcement.
According to TamilNet, a pro-LTTE website, the Sri Lankan Air Force launched a series of bombing raids on the zone shortly after the government’s announcement at midday. It also claimed Navy gunboats had shelled the area.
Mr Holmes, UN Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator, had suggested that earlier government pledges had not been honoured.
"I hope it (the government declaration) will be genuinely respected this time. It has not happened in the past," he said.
He said tens of thousands were still being held by the LTTE, which was refusing to allow them to leave, or caught between ongoing fighting between the Tigers and Sri Lankan security forces.
He said was "greatly concerned" at the reports of continued shelling and stressed the need for humanitarian agencies to be given access to the zone to help civilians facing "critical levels of hunger." Human Rights Watch last night called for both sides to be investigated for war crimes.
Brad Adams, Asia director at Human Rights Watch, told the Daily Telegraph: "We are calling for an investigation into possible war crimes by both the Sri Lankan government and the Tamil Tigers.
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