The UN High Commissioner for Refugees office said it was alarmed by the situation of Serbs in Kosovo, forced to flee by ethnic Albanian gangs using tactics similar to those used by Serbs earlier this year.
"Conditions for those who remain have noticeably worsened over the last few weeks, with increasingly violent attacks on the rise," a UNHCR spokesman, Ron Redmond, said.
"Old women are now being shot through the doors of their apartments - including two women in their 70s within the past week."
Dennis McNamara, the UNHCR's top official in the Balkans, said Serbia was now home to nearly 700,000 refugees who had fled regional conflicts this decade.
"Perpetuating the refugee cycle is going to destabilise the region," said Mr McNamara, who is deputy to Bernard Kouchner, the UN mission chief in Kosovo.
Nato's air strikes were designed to stop the mass expulsion of ethnic Albanians - the great majority in Kosovo by Serbs. But the alliance now seems powerless to stop Serbs in turn being driven out.
UNHCR estimates at least 170,000 Serbs - out of a population of 180,000-200,000 - have fled Kosovo since K-For and the UN arrived in mid-June, Mr McNamara said.
The previous size of the Serb population in Pristina - which has about 250,000 inhabitants - is disputed. But a drop to 1,000-2,000 in a few months is dramatic.
Mr Redmond said expulsions of Serbs appeared systematic, but UNHCR did not know who was behind them. "A disturbing pattern has arisen in the method of intimidation used against Serbs still in the city," he said.
"First a warning letter is received ordering them to leave their homes, then the threat is delivered in person, followed a few days later by physical assault, in some cases even murder... [Serbs'] freedom of movement is virtually non-existent."
Refugee agency officials say entire blocks of flats may be cleared by ethnic Albanian gangs, who first force Serbs to sign documents transferring ownership of their homes.
Nato peacekeepers said yesterday that they had arrested 78 people in the past 24 hours.
