Sri Lankan President Mahinda Rajapaksa appealed Friday to the Tamil Tiger insurgents to allow the estimated 250,000 civilians trapped in the country's northern war zone to flee to safety in the next 48 hours.

Rajapaksa's appeal, published Friday on a government website, came after human rights groups and a top UN official expressed serious concern for the fate of the noncombatants in the area amid reports of heavy civilian casualties.

Human rights groups have accused the Tigers of holding the civilians hostage and accused the military of launching heavy attacks in areas filled with noncombatants, including a government-declared "safe zone" in the north. The insurgents and the military deny the charges.

Rajapaksa said the insurgents' refusal to let the civilians leave was endangering their lives and he accused the militants — known formally as the Liberation Tigers of Tamil Eelam — of putting their heavy artillery inside the "safe zone" and using it as a "launching pad" for attacking government troops.

"I urge the LTTE, within the next 48 hours, to allow free movement of civilians to ensure their safety and security. For all those civilians, I assure a safe passage to a secure environment," he said.

The Tigers could not be reached for comment since nearly all communication to the north has been severed. Independent accounts of the fighting are not available because most journalists are barred from the war zone.

UN 'seriously alarmed'

UN High Commissioner for Human Rights Navi Pillay said top UN officials were "seriously alarmed" over the fate of the civilians in the north.

"It seems there may have been very grave breaches of human rights by both sides in the conflict, and it is imperative that we find out more about what exactly has been going on," she said.

She also expressed concern over the government's ban on letting aid workers and reporters enter the conflict zone.

"The lack of access for independent monitors, humanitarian workers and the media only adds to concerns that the situation may be even worse than we realize," she said.

The insurgents, who have been fighting for a separate homeland for ethnic Tamils in northern Sri Lanka since 1983, were ousted from all major towns after heavy battles in recent months and are now cornered in a 300-square-kilometer area of jungle and villages in the northeast.

The Red Cross estimates 250,000 civilians are trapped in the shrinking territory still under the Tigers' control.

"The overall humanitarian situation remains precarious for thousands," the Red Cross said in a statement. "Stocks have been depleted, and sustainable ways of producing food locally have become almost nonexistent."

Convoy held up

On Thursday, the United Nations evacuated hundreds of wounded civilians from the conflict zone.

UN spokesman Gordon Weiss said a convoy transporting the injured people was held up by Tiger militants Wednesday before being allowed to cross into government territory Thursday morning.

It included "50 critically injured children on board and some hundreds of [other] critically injured," he said.

Military spokesman Brigadier Udaya Nanayakkara has said no civilians have been killed, but that some people who were forced by the insurgents to build fortifications might have been wounded in crossfire.

Amnesty International said both sides appeared to be violating the laws of war by endangering civilians.

Tamils have suffered decades of marginalization at the hands of governments controlled by Sri Lanka's Sinhalese majority. More than 70,000 people have been killed in the civil war.