The past week has seen the most serious violence in the region since last November, when four Serb policemen were killed by suspected Albanian guerrillas fighting for the independence of three Albanian-populated towns near the Kosovo border.

UN special co-ordinator for the Balkans Carl Bildt told the BBC that the Presevo situation was "the most serious threat to stability in the Balkans, and there is very little awareness or attention to what is about to explode".

UN concern

The BBC's Nik Gowing in Davos has been shown evidence by foreign diplomatic sources that the guerrillas now have several hundred fighters in the 5km-deep military exclusion zone on the boundary between Kosovo and the rest of Serbia.

The sources said that:

• Certain Nato-led K-For forces were not preventing the guerrillas taking mortars and other weapons into the exclusion zone

• The guerrilla units had been able to hold exercises there, including live-firing of weapons, despite the fact that K-For patrols the zone

• Western special forces were still training the guerrillas, as a result of decisions taken before the change of government in Yugoslavia

• Guerrilla leaders had now taken over from political leaders in many Albanian villages within the zone

• They now controlled the heights overlooking the villages of Presevo and Bujanovac

Our correspondent says Yugoslav officials are concerned that Albanian actions may reach the point where the Yugoslav military - which currently supports Mr Kostunica - will insist on entering the security zone, creating another Albanian refugee crisis.

Abrupt departure

Mr Kostunica had been due to speak to the Davos forum on plans for the economic regeneration of Yugoslavia, before his abrupt departure.

In an exclusive interview with BBC World Television, Mr Kostunica said: "The threat is still under control, but it can get out of control. There, we have once again the flames of a new war... we are trying by all means to prevent it.

"The situation is getting worse and worse," he said.

He told the BBC the seriousness of the situation meant that, as president, he should be in Yugoslavia rather than the Swiss Alps.

The Yugoslav Government has demanded an urgent United Nations Security Council meeting to discuss the guerrillas' actions.