Tens of thousands of civilians have fled their homes in northern Sri Lanka in recent weeks as the military ramped up its offensive against the Tamil Tiger rebels' heartland, international aid groups said.

New fighting between the two sides killed 27 rebels and two government soldiers, the military said yesterday.

For months, the two sides faced each other over relatively static front lines, but the military broke through in the past few weeks, capturing a series of rebel bases and large chunks of territory.

Between 55,000 and 75,000 people have been displaced by the fighting over the past 2 1/2 months and as many as half of them are living under trees and tarps, said Neil Buhne, the United Nations' country representative for Sri Lanka.

The International Committee of the Red Cross said Thursday that health authorities are struggling to provide basic needs food, water, shelter and sanitation to those displaced. The escalating fighting also is making it difficult for humanitarian workers to get access to some areas, the Red Cross said.

Initially, many of the displaced moved in with relatives, but as the fighting progressed and many were forced further from their villages, that support network deteriorated, Buhne said.

While some have found temporary shelter in 17 schools in the area, others have no place to go, he said.

So far, aid groups have been able to get food and needed supplies to the displaced families, Buhne said, but he cautioned the situation could quickly worsen.

"If there's not continued access of humanitarian supplies, if civilians are not protected, if they are limited in where they can move, it's something that could develop into a crisis," he said.

A rebel spokesman could not be reached for comment. However, the Tamil Tigers said in a statement they fear many of those seeking shelter in schools might be forced to leave when students return from vacation next month. The rebels also have called on residents to dig bunkers in their homes and workplaces to protect against military attacks.