Nato scrambled to deploy up to 1,000 additional troops to boost the 17,000-strong Nato-led Kfor peacekeeping force in an attempt to clamp down on renewed ethnic violence.

Serbian Orthodox churches were burnt down in Kosovska Mitrovica and Vucitrn, while the UN police headquarters in the town of Prizren was also attacked. Smoke billowed from Serb houses set ablaze in the mixed town of Kosovo Polje, and burnt-out cars littered the streets of Pristina. UN troops and police came under sustained gunfire as they attempted to rescue beseiged Serbs.

At least 22 people have been killed, and more than 500 injured in the worst outbreak of fighting since the Nato air-strikes in spring and summer 1999. All the deaths came in gunbattles, riots and street fighting on Wednesday.

Kfor troops were last night battling an armed gang of hundreds of Albanians attempting to break into the Serb village of Caglavica just outside Pristina, the provincial capital. In the town of Obilic Nato troops evacuated 100 Serbs after their houses were set ablaze.

There were suspicions that the attacks on the Serb communities were timed to coincide with the fifth anniversary of the Nato airstrikes against Serbia, which began on March 24, 1999. The attacks appeared to be closely co-ordinated.

Speaking from Pristina, Derek Chappell, a UN police spokesman, said: “We have seen many acts of violence in the last four years. We have not seen a co-ordinated action, with this level of violence, when thousands of people from all regions of Kosovo attack Serbs, Serb property and Serb symbols such as churches, all on the same day. The targets are very specific.”

Mr Chappell said: “It is difficult to think that all this is spontaneous, although there is no evidence to link these events to any organisation.”

The violence triggered fears that Kosovo could once again descend into war, possibly dragging in Serbia and destabilising the whole of the southern Balkans.

Talks are due either this year or next to resolve Kosovo’s final status, but the recent fighting shows that any hopes of reconciliation are unlikely to be met in the near future. Flights in and out of Kosovo were suspended and internal boundaries with Serbia were closed. UN troops blocked the road between Pristina and Mitrovica, checking travellers. Tension soared on the internal border between Serbia and Kosovo and the Interior Ministry put paramilitary police on top alert.

Vojislav Kostunica, the Serbian Prime Minister, said the violence had been “planned and organised” by ethnic Albanians bent on driving the remaining Serbs from the province. He urged the UN Security Council to deter “ethnic cleansing”.

Ethnic violence also spilled over into Belgrade, the Serbian capital, where demonstrators reacted with fury. Hundreds of rioters tried to set fire to a mosque there on Wednesday night, and a mosque was also set ablaze in the southern city of Nis.

More than 10,000 demonstrators rallied in central Belgrade around government buildings, blocking main roads. The crowd shouted: “We’ll go to Kosovo”, and “Serbia arise”, the news agency BETA reported.

Even during the wars, Belgrade’s mosque remained a comparative haven. Observers often noted how in the cosmopolitan capital all of Yugoslavia’s nationalities could live together, whether Orthodox, Catholic, Muslim or Jewish.

Ordinary Belgrade people were dismayed at the attack on the mosque. “This is a disgrace, a disaster,” said one middle-aged woman looking at the damage. “The police should have stopped them.”

The US embassy was also a target. Police used tear gas and stun grenades to scatter protesters trying to smash its facade. They said 50 people were injured, mostly police.

Protesters broke the glass on the Islamic Community building in Novi Sad, and police in the north Serbian towns of Sombor and Apatin reported attacks on the property and homes of Albanians.

Nato played down the prospects of renewed military conflict, saying the alliance and the United Nations were committed to keeping the peace. Jamie Shea, Nato spokesman, said: “I don’t believe there is a possibility of a war.”

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