The convoys, with aid for about 11,000 refugees, were to have set off on Monday.

But the separatist Kosovo Liberation Army and other ethnic Albanian villagers have reported renewed shell-fire.

Paul Welsh: Despite cease-fires, Kosovo still far from stable

There have also been reports that a regular Yugoslav army unit with tanks and artillery is moving from the provincial capital, Pristina, into an area of Kosovo where three Serbian policemen were killed on Saturday.

A spokeswoman for the UN refugee organisation in Kosovo said: "We had two convoys scheduled for this morning, one to Peterac and the other to upper Streoc, but since there were unconfirmed reports of heavy shelling in those areas we decided to cancel the convoys."

Separatists allege they came under heavy fire

By the terms of the Belgrade agreement, signed last week by the Yugoslav President, Slobodan Milosevic, the Serbs must withdraw their forces or face Nato air-strikes.

An unarmed international monitoring force is to be sent to the province to verify that both sides abide by the peace agreement.

But the Organisation for Security and Co-operation in European, in charge of the 2,000-strong monitoring force, said it is still assembling its people and will not be going to the area to confirm the reports of fighting.

However, Nato Director-General, Javier Solana, said the alliance had begun air surveillance, and vowed to prevent the violence from spreading.

"We will not allow this to happen. This must not lead to a resumption of violence," he said.

On Friday, Nato gave President Milosevic an additional 10 days to comply with UN demands, which extends the deadline for Serb withdrawal to 27 October.

Slow progress

One of a convoy of Serb vehicles leaving Kosovo

The United States and Nato have reported some progress in the withdrawal of Serb forces, but say the numbers still exceed the level required by the Belgrade agreement.

It is unclear whether the Serb unit reported to be moving to the Drenica region would be breaching UN demands or not, because it is said to be based in the region.

Jackie Rowland: Despite fears, refugees starting to come down from the mountains

While there are reports of more ethnic Albanians taking fright and fleeing, our correspondent says some of the estimated 270,000 refugees who were displaced in the fighting have been returning to their towns and villages.

But both sides have been accusing each other of continuing the hostilities.

US envoy Richard Holbrooke says Nato planes remain on alert

The Serbian government says the KLA is stepping up attacks to scuttle last week's peace agreement.

After the Serb policemen were killed over the weekend, the head of Kosovo's regional government, Veljko Odalovic, said: ''These latest attacks show that the Albanian terrorists are trying by all possible means to prevent the realisation of the agreement.''