The United Nations refugee agency says customs tolls imposed by the Macedonian authorities are jeopardising its aid operation in Kosovo.

Trucks and rail wagons carrying thousands of tonnes of aid and building materials have been stranded at the border between Macedonia and Kosovo since the charges were imposed last month.

The agency is refusing to pay the tolls -- worth two-hundred thousand dollars a month -- as a matter of principle, saying humanitarian deliveries are normally exempt from customs charges.

But a BBC correspondent at the agency headquarters in Geneva says that with most ethnic-Albanian refugees back in Kosovo, the need for aid to rebuild their homes before the onset of winter is crucial.