- guardian.co.uk, Friday 1 December 2000 15.31 GMT
A small band of ethnic Albanian rebels have seized control of hilltop positions along part of the border.
In the past few days the rebels have stepped up their attacks, killing four Serb policemen.
The area is home to much of Serbia's ethnic Albanian minority, many of whom live in the demilitarised zone set up at Nato's insistence at the end of last year's war.
The Yugoslav army is barred from the demilitarised zone, which is patrolled by lightly armed Serb police, vulnerable to guerrilla ambush.
The rebels call themselves the Liberation Army of Presevo, Medvedja and Bujanovac: the main Albanian population centres in southern Serbia.
They are likely to be cadres of the Kosovo Liberation Army which fought the Yugoslav army before Nato's army took over the province.
Since the arrival of the Nato forces the KLA has splintered. Denied a primary role in the government or in Kosovo's fledgling police force, it has also been outflanked by the leading political moderate, Ibrahim Rugova.
If the guerrillas provoke the Yugoslav military into returning to the buffer zone, their soldiers will come face to face with Nato troops, who are obliged to prevent any Serb military incursion into the buffer zone.
The new leader in Belgrade, Vojislav Kostunica, has asked for calm and a political solution to the violence.
However, he is insisting that Nato take control of its side of the border and stop the guerrilla incursions.
Useful links
K-For
Map of the region
The war
Chronology of events in the Balkans
Ethnic cleansing
Critique of the Kosovo Liberation Army
