The co-operation protocol will help EULEX and Serb police share information on cross-border organised crime and is a pre-condition for Serbia to obtain visa free travel to the EU in 2010.

Kosovo leaders said that EULEX' direct dealing with Serbia undermines their attempt to establish a sovereign state.

"The Kosovo leaders reiterated in the meeting their firm position against the protocol and emphasised that from today any debate and discussion on this issue is completely closed. Kosovo does not take any obligation and responsibility for issues which it has not decided in a sovereign way," the office of Kosovo president Fatmir Sejdiu said.

The statement came out after Mr Sejdiu and Kosovo prime minister Hashim Thaci broke off talks with EULEX chief Yves de Kermabon and the EU's civilian representative to Kosovo, Pieter Feith.

The police protocol has stoked anger in the majority ethnic Albanian population in Kosovo.

On Wednesday, the ethnic Albanian Vetevendosja ("self-determination") movement attacked EULEX vehicles in events leading to 21 arrests.

"We want the Republic of Kosovo to join the EU. But what we need are economic experts, doctors, scientists to help us develop. Not EU policemen to rule over us in a completely unaccountable way," Vetevendosja leader Albin Kurti told EUobserver.

Mr Kurti said Serbian police were involved in the killings of ethnic Albanian civilians in the 1990s: "They are criminals. They killed 12,000 people and only a dozen or so of those responsible are in prison."

Serbia's minister for Kosovo, Goran Bogdanovic, gave provocative comments to the Serbian Vecernje novosti newspaper on Thursday.

"With this document [the police protocol], the EU is confirming Serbia's integrity even on the areas that our country does not have full control over," he said.

Kosovo declared independence from Serbia in February 2008 with the backing of the EU institutions and the US. Twenty two out of 27 EU states have recognised its sovereignty. But Spain, Greece, Cyprus, Slovakia and Romania have not.

The EU visa free deal will cut along ethnic lines in the Balkans.

The agreement is to embrace the majority Orthodox Christian countries, Serbia, Montenegro and Macedonia. But it will exclude the majority Muslim Kosovo and Albania.

Bosnian Muslims will also be stuck with visa requirements. But most Bosnian Serbs will benefit from the EU deal because they hold Serbian passports.