"We suggested the idea of some kind of humanitarian pause to allow that to happen and to allow the civilian population to leave," he said.
"This is an extremely worrying situation and therefore, our first appeal is to the LTTE to let the civilians out in a safe and orderly fashion."
The government says its forces have confined the LTTE to an area of about 21sq km, most of which is a government-declared safe zone, on the island's northeastern coast near the town of Puthukudirippu.
Rosemary DiCarlo, the US representative to UN for special political affairs, expressed "real concern" on Thursday over the increasing death toll and condemned the LTTE's use of civilians as human shields.
"We call on them to lay down their arms, renounce violence and negotiate with the government."
DiCarlo also criticised the Colombo government for continuing to shell areas heavily populated by civilians in its efforts to end a 25-year civil war by subduing the LTTE.
"We are very concerned that the government of Sri Lanka continues its shelling in areas where there are large numbers of civilians, very close to hospitals and civilian facilities, we understand quite a number of civilians have perished because of these attacks," she said on Thursday.
"We've had promises, but we need to see results."
But the Sri Lankan military has rejected the claims that it had fired on civilians.
Brigadier Udaya Nanayakkara, the spokesman for Sri Lanka's defence ministry, told Al Jazeera on Friday that government forces have been taking every precaution against loss of innocent life.
He said that, contrary to the claims, thousands of civilians have sought the military's protection from the fighting, with more coming daily.
Hews Palihakkara, Sri Lanka's ambassador to the UN, said the government had declared a 48-hour ceasefire period but the LTTE was preventing civilians from leaving.
"If the LTTE is ready to let the civilians go today, I will persuade my government to agree to any modality. You can call it a pause or something else," he said.
A senior LTTE commander has also denied using civilians as human shields and rejected government claims that they were being kept hostage in the combat zone.
|The LTTE has denied claims it is using civilians as human shields [Reuters]
In a statement smuggled out of Sri Lanka to Al Jazeera, Ilampirathi, an LTTE "lieutenant-colonel" in Puthukudirippu, said the group was not stopping Tamil civilians from leaving.
"It's a false allegation by our enemies. Our people are determined and continue to stay here despite the attacks from the enemies," Ilampirathi said, responding to questions posed by Al Jazeera and filmed last month.
"Our people are helping the fighters in every way possible to fight the Sri Lankan forces.
"They say they have to fight if they are to survive."
Media access to the war zone is tightly controlled and claims by either the Sri Lanka government or the LTTE cannot be independently verified.