Addressing journalists at the UN in New York, Mr McNamara said aid organisations would be unable to carry out the repairs and building work needed to house everyone.

Some people have been able to start rebuilding their houses

"It will certainly be a tough winter for many Kosovars, I'm afraid, yet again," he said. "Some of us believe it might get more difficult before it gets better in Kosovo."

He estimated that "at least 300,000 people" would spend the winter living in the homes of other people, in tents or elsewhere.

Mr McNamara - deputy special representative in Kosovo's UN administration - warned the 800,000 or so refugees that, contrary to their expectations, there would be no major programme of reconstruction before winter.

Repair kits

More than 100,000 houses were damaged or destroyed during Serbia's offensive against Kosovo's Albanian majority, which ended after a Nato air campaign earlier this year.

Kosovo Polje: Continuing ethnic tensions

Mr MacNamara said the UN and other agencies would provide temporary repair kits capable of patching up one basic room in 50,000 damaged houses.

Such basic measures would enable an estimated 350,000 people to spend the winter indoors.

"The other 300,000 to 400,000 people whose homes have been completely destroyed will have to continue being accommodated with host families, relatives and friends, in tents and elsewhere during the winter," he said.

"We're in the middle of a very difficult and complex phase in this operation, and I think many of us also realise winter will be an extremely testing time for the entire operation," Mr McNamara said.

Ethnic tension

Mr McNamara said he was also worried by the number of revenge attacks by ethnic Albanians on local Serbs in the region.

Two Serbs died in a rocket grenade attack on Tuesday in the province's southern town of Kosovo Polje.

Some gypsies have already fled Kosovo, fearing violence from Albanians who accuse them of siding with the Serbs.

"Attacks continue on a regular basis against Serbs in Kosovo and - less acutely but consistently - the Roma gypsy population," Mr McNamara said.