(Click here to see a map showing major Serb population centres in Kosovo)

It said more than 13,000 Serbs had left through Montenregro, Serbia's smaller partner in the Yugoslav federation.

UNHCR: "More than 15,000 fled since last Wednesday"

The agency said it would continue working with K-For "to ensure that people of all ethnicities will be protected".

But the Serb exodus is evidence that they have little or no confidence in K-For's presence.

Many Serbs fear that K-For will not - or cannot - protect them from guerrillas of the Kosovo Liberation Army (KLA), who have been coming down from the mountains to return to their towns and villages.

Whole neighbourhoods on fire

On the outskirts of Pristina, Serbs torched dozens of their own homes before joining convoys of cars, lorries and tractors heading for Serbia.

Fear is felt by both old and young

At Kosovo Polje, a mainly Serb town south-west of Pristina, whole neighbourhoods were on fire. There have been allegations that KLA fighters burned many of the homes.

Some of the civilians have joined military convoys - their vehicles, loaded with household goods and even farm animals, interspersed among tanks and camouflaged lorries.

"Activities by terrorist gangs"

Orla Guerin in Kosovo sees Nato troops persuade KLA fighters to disarm

The Yugoslav authorities have denounced what one minister called "activities by terrorist gangs" - a reference to the KLA - for causing the exodus.

The Minister for Refugees, Bratislava Morina, called on K-For to protect Serb homes; their mandate, she said, was to safeguard Serbs and Montenegrins, not to expel them.

The UNHCR says it fears a repetition of the same pattern of displacement of Serbs seen in December 1991 in Western Slavonia, in August 1995 in the Krajina and in Sarajevo after the December 1995 Dayton agreement.