(Adds U.N. statement, paragraph 5)

PRISTINA, Feb 19 (Reuters) - U.N. police pulled out from a Kosovo border post that was destroyed on Tuesday by Serbs who vowed never to submit to the authority of Kosovo's Albanian government and its Western backers.

Reuters witnesses saw police destroy official documents and remove computers then leave in a convoy escorted by special riot police in armored cars, and the border post known as Gate 3-1 north of the town of Zubin Potok was abandoned.

It was one of two border posts between Kosovo and Serbia attacked and set on fire by Kosovo Serbs earlier in the day in the Serb-dominated northwest corner of Kosovo.

Kosovo has declared its independence from Serbia, a move recognized on Monday by the United States and major European Union powers but strongly opposed by Serbia and Russia.

Police manning both posts had called for help from the NATO peacekeeping force, KFOR, a 35-nation force of 17,000 which has French, Belgian, U.S. and Danish troops in the border zone.

A statement by the U.N. mission in Kosovo made available in New York quoted U.N. special envoy Joachim Ruecker as saying the attacks were "completely unacceptable and will not be tolerated." Ruecker said he had asked U.N. police and KFOR to restore order at the border crossings.

The mission indicated that the police planned to return to the crossings, saying the two posts involved had been closed "for 24 hours."

The attacks highlighted the challenge facing a European Union law-enforcement mission preparing to deploy in the Albanian-majority territory which has been under U.N. administration for nearly nine years.

Asked if the EU was prepared to call on NATO to enforce the authority of its mission in Kosovo's Serb stronghold, EU foreign affairs chief Javier Solana said the 2,000-man mission was not deployed yet.

"Don't ask for the mission to do something today that they are not in a position to do," he told a news conference on a visit to Pristina. "I would like to say that it will be deployed in the territory of Kosovo, in all of Kosovo."

"KFOR is here and KFOR has used its responsibility, its obligations, already today."

One Western official in Kosovo said it looked as if the province was close to effective partition.

Kosovo Prime Minister Hashim Thaci played down the attack on the border posts.

"Everything is under the control of the NATO authorities, Kosovo police and the United Nations, and no isolated incident will undermine Kosovo's independence celebrations," he said.

"Kosovo is integral, inseparable and Kosovo territory is guaranteed and recognized internationally." (Additional reporting by Louis Charbonneau at United Nations) (Reporting by Matt Robinson and Douglas Hamilton; editing by Richard Meares and Cynthia Osterman)