Thousands of civilians fleeing fighting in Sri Lanka have been interned by the government in cramped, makeshift camps with overflowing drains, water shortages and the threat of disease looming large in the sweltering, unsanitary conditions.

The government allowed a small group of international journalists into Menik Farm camp, which is regarded by aid agencies as by far the best-equipped of the camps. But even here, people complain bitterly about their treatment and the lack of freedom of movement.

In the camp people are herded behind coils of razor wire strung out between sharp wooden stakes and are not allowed to leave. Rows of tents stretch as far as the eye can see.

Those who have been given shacks made of sheet metal bake in the scorching heat. Soldiers armed with automatic rifles patrol inside and outside the camps. "This is a jail," one woman told the Guardian.

As Sri Lanka's army pounds a no-fire zone where the last remnants of the Tamil Tigers are cornered, its government says it expects the tens of thousands of civilians still trapped alongside the rebels to start to make their way to safety in the next few days. But while it had attempted to portray the decision to place displaced civilians in the camps as a humanitarian gesture, the woman in charge of one of them yesterday admitted that the intention was to keep those inside shut off from the rest of the population.

Piency Charles, the government agent at Menik Farm, near the town of Vavuniya, said the authorities believed that Tamil Tiger fighters had entered the camps alongside civilians.

"Our concern is the security of these people and the security of the whole country. If we release people from here they may cause damage outside," she said.

Sasi Kumar fled here with his fmaily from Kilinochchi, the former Tamil Tiger stronghold, weeks ago as fighting intensified around the city. They were promised security and food, but were not expecting what they found. They sleep on mats on the bare concrete floor in a tin-walled hut that soaks up the heat of the sun.

Kumar, 34, gestured at his two-month-old baby, Vigee, sleeping fitfully on the floor surrounded by flies.

"It is very hot and the health problems are very bad. The children have coughs and diarrhoea," he said.

"We can't go out, we have to stay inside the fences. Ask the forces why they brought us here," he said angrily. "They say we can only go back when it is safe, maybe in a year."

Gordon Weiss, the UN spokesman in Sri Lanka, said of the camps: "It is a nasty place to be and it is distinctly uncomfortable."