The conflict in the Presevo region is part of the legacy Yugoslavia's new democratic administration has inherited from ex-president Slobodan Milosevic's regime.

At the end of Belgrade's war with Nato over Kosovo in 1999, Mr Milosevic had to agree not only to the withdrawal of his security forces from the province, but also to the establishment of a 5km-deep demilitarised zone along Kosovo's boundary with Serbia.

The Yugoslav army is watching closely

This buffer zone provided the opportunity for ethnic Albanian militants to launch an armed campaign against Serbian rule.

With Yugoslav army and Serbian special forces excluded from the area at that time, a guerrilla force, known as the Liberation Army of Presevo, Medvedja and Bujanovac, emerged last year in these three municipalities which have a substantial ethnic Albanian population.

KLA successor

Modelled on the Kosovo Liberation Army - which was disbanded after the war in Kosovo - the new guerrilla movement has been able to operate with relatively little concern about retaliation from Belgrade, which has been allowed to keep only the local police force in the area.

The UCPMB says it is acting in self defence

As the casualty toll has mounted the multinational peacekeeping force in Kosovo, K-For, has stepped up its patrols along the border between Kosovo and the Presevo valley in a bid to cut down the scale of cross-border movements - both of guerrillas and supplies.

Initially, the guerrillas' publicly-acknowledged objective was to protect the local ethnic Albanian population of some 70,000 people from the repressive actions of the Serb security forces.

At one stage during the clashes several thousand ethnic Albanians fled to Kosovo - either to avoid Belgrade's crack-down or to escape the fighting.

Uniting with Kosovo

Since Mr Kostunica's victory the Serbian police have adopted a much less hostile attitude towards the local Albanians in a bid to win the hearts and minds of the inhabitants.

But as the guerrilla force has become better established, its leaders have started to talk about more far-reaching goals, first and foremost, of uniting their region with Kosovo.

President Kostunica wants Nato to clamp down on rebel activity

That raises considerable concern among Serbs, who are already fearful that Kosovo - currently under a United Nations administration - is moving towards independence.

The last thing they would want to see is Presevo - an area that's internationally acknowledged to be an integral part of Serbia - joining Kosovo Albanians in their bid for independence.

But concern goes well beyond Serbia.

The Presevo valley is strategically important insofar as it is located on the northern border of the Former Yugoslav Republic of Macedonia.

Fears of larger conflict

Macedonian Slavs are worried about their own militant ethnic Albanians who harbour the desire for a greater Albanian entity that would bring together Kosovo, northern and western part of Macedonia and the Presevo valley.

Later on that state could potentially even merge with Albania itself.

In the Presevo region - a small area with a small population - the guerrillas number just a few hundred fighters.

But Belgrade and Skopje were concerned that while Serbian forces were barred from Presevo, the localised clashes could one day reignite a larger conflict.

That is why President Kostunica repeated his calls for Yugoslav and Serbian forces to be allowed back into the buffer zone - or, at the very least, for the zone to be reduced in size.

Initially the UN was reluctant to accept this request - partly because it feared that such a move would lead to an escalation of the fighting, and partly because it wanted to avoid possible border incursions which could trigger clashes between Yugoslav forces and K-For.