The separatist Liberation Tigers of Tamil Eelam (LTTE) have forcibly recruited children and a U.N. worker, and are shooting and killing people trying to flee Sri Lanka's war, the United Nations said on Monday.

"They (people) should not be shot at and they are being shot at by the LTTE," Neil Buhne, the U.N. resident coordinator for Sri Lanka told Reuters. He did not say how many had been killed.

"The LTTE is now recruiting people as young as 14. It had been happening but it seems to be accelerating."

The Tigers could not be reached for comment.

Sri Lankan troops have cornered the rebels in a tiny swathe of jungle and are battling to finish off a separatist rebellion that has raged off and on since 1983 and is now one of Asia's longest-running wars.

Buhne said the LTTE was actively keeping people from leaving a war zone now measuring just 142 square km (55 sq miles).

Amid growing calls to protect thousands of civilians trapped by the LTTE, the military established a no-fire zone inland, and moved it to the coast last week, saying the LTTE had fired artillery from the original one and forced people out of it.

There were also reports civilians had been killed in fighting inside a newly demarcated no-fire zone on the Indian Ocean island's northeastern coast, the U.N. said.

"Reports from yesterday indicate that there was some fighting inside the zone. This fighting led to the deaths and injury to yet more civilians," the United Nations said in a statement, without assigning blame nor saying how many were killed.

However, the military's spokesman, Brigadier Udaya Nanayakkara, said there was no fighting on Sunday in the new no-fire zone.

"There is no resistance from the area, and there are no confrontations there," he said.

The new no-fire zone had provided some respite to people but needed to remain free of any combat, Buhne said, urging both sides to respect its boundaries.

Buhne said the LTTE was further pressuring civilians already traumatised by fighting, who are now stuck with minimal food and no medicine.

He also said the LTTE forcibly recruited one of the 15 local U.N. employees into their ranks. Those 15 were physically barred from leaving the war zone last month by the LTTE, along with 75 dependents including 40 children, he said.

On Monday, the International Committee of the Red Cross ferried out by boat 400 people including sick and wounded who were in the no-fire zone, its third evacuation from the area in a week.

Buhne has just returned from refugee camps in the north-central town of Vavuniya, where those who fled the war are being housed in government-run camps guarded by the military.

"If you look at the arrangements to receive people there, I think they are positive," Buhne said.

Most people he talked to had moved 10 or 15 times as fighting swept across the island's north, taking refuge in trenches or bunkers for weeks or days at a time.