Albanian police in the village of Krume, just a few kilometres from the frontier, said information about the tanks' progress came from KLA guerrillas, who had been operating in the vicinity of the tanks.

There have also been reports that Serbian shells hit Krume and international monitors say shelling destroyed an Albanian border post on Monday.

The Pogaj post, only 500m from the main refugee crossing point at Morino, was "levelled to the ground," according to a spokesman for the Organisation for Security and Cooperation in Europe (OSCE), which monitors the border situation.

The UNHCR is evacuating refugees from camps in the area amid fears that they are in danger of being shelled by Serbian forces.

The reports of Serbian border movements followed heavy fighting between the KLA and Serbian forces there on Monday.

The BBC's correspondent in Kukes, Caroline Wyatt, says the situation is becoming increasingly tense as the KLA tries to clear a supply corridor along the Kosovo-Albanian frontier.

Albanian villagers watch Serbian shelling near their village

The escalation in KLA activity has brought a sharp response from Serbian forces.

Nato warplanes bombed Serbian hillside positions on Monday, nearly hitting the Serbian border checkpoint at Morino.

Nato has previously denied co-ordinating its air campaign with KLA activities, but Monday's strikes appeared to be designed to support KLA guerrillas on the ground.

Refugees under threat

Serbian shells hit an area less than a mile from the Morino crossing point, which has been used by the hundreds of refugees fleeing into Albania every day.

An injured KLA guerrilla recovers in a refugee camp

Aid agency representatives in the town of Kukes near the border have warned that the area is becoming increasingly unsafe.

Speaking from the Morino checkpoint, UNHCR representative Rupert Colville said the incoming refugees were like "walking ducks if anyone wants to take a potshot at them".

"There's been a marked deterioration of security along the border, and it's very disruptive," Mr Colville added.

The UNHCR is trying to speed up the evacuation of refugees from camps in and around Kukes, which are well within the range of Serbian artillery fire.

Around 2,000 refugees are being moved to camps further south every day, but many remain reluctant to move away from the border and further from home.