WFP spokeswoman Lindsay Davis said refugees were reporting that Serbian forces were destroying their food supplies in order to drive them out.

The UN agency says it has also been getting repeated reports that in some parts of Kosovo the police are preventing those with Albanian names on their identity cards from buying food.

Shops looted

Lindsay Davis of the UN World Food Programme: "Only the Serbian population are allowed to buy food"

"Any Albanian shops that are operating are being trashed and being looted by the Serbian forces," Ms Davis said.

The BBC correspondent in Macedonia, Paul Wood, says it is unclear how widespread or reliable these allegations are. Refugees he spoke to did not make this complaint.

Many of the camps are already badly overcrowded

But there is a growing catalogue of attrocities reportedly committed by the Serbian military, including reports of mass rape and sexual molestation of women.

Meanwhile, an estimated 5,000 more refugees have arrived in Macedonia putting further strain on already overcrowded camps.

The BBC's Rageh Omaar: "There are three times as many people here as there should be"

A spokeswoman for the UN refugee agency said the camps were full beyond breaking point and another aid worker told the BBC the crisis was the worst he had seen in 37 years of humanitarian work.

Thousands more expected

Steve Bullock, President of the American Red Cross: The scale of the disaster caught us off guard

A new camp was to open today in Cegane in the west of the country but is not expected to cope even with refugees who have arrived in the last week.

Tens of thousands more refugees are thought to be on their way.

Thousands more are expected to cross the border

Already displaced Kosovars are thought to make up something approaching 10% of Macedonia's population and state officials have expressed fears about the political consequences of upsetting the country's ethnic balance.