Guns are not allowed inside the UNHCR tent

"There is a general panic. People are worried about the proximity of the National Liberation Army (rebels) and they think fighting will start," she told BBC News Online.

Fewer people fled from Macedonia to Kosovo on Tuesday than on Monday - 2,500 compared with 5,300 - but the UNHCR remains concerned.

Officials say the population of Skopje fears both direct military action - after threats from rebels based in Aracinovo on the city outskirts - and the possibility of inter-ethnic violence.

On Wednesday, police were reported to be registering young men in Skopje suburbs, issuing some with uniforms and assault rifles, though without bullets.

Click here to see map of the region

"If there is another incident it will cause another big stream of refugees and there is the potential for a large exodus," said Ms Van Genderen Stort.

"There have been threats to the airport, and people are worried that this is going to come true."

The exodus over the last few days is roughly half of the total of 41,000 to have entered Kosovo since the conflict between the rebels and Macedonian Government forces began in February.

A refugee weeps as he leaves Macedonia

The vast majority are members of the country's ethnic Albanian minority, who comprise up to a third of the total population.

They have been offered refuge by Kosovans, many of whom were themselves in Macedonia in 1999.

The UNHCR says at least several thousand more can be absorbed in the same way.

"There is a still a willingness of the Kosovo population to put them up, to repay the debt of 1999. This is a wonderful support for us," says Ms Van Genderen Stort.

However, some of those entering Kosovo are members of the Macedonian majority population fleeing to Montenegro.

The UNHCR estimates that some 18,000 people have been displaced inside Macedonia in the four months of fighting.