By Ellie Tzortzi

BELGRADE, Dec 6 (Reuters) - The United Nations and European Union must guide Kosovo on a path to independence as soon as failed negotiations with Serbia officially end on Dec. 10, a leading think-tank said on Thursday.

Failing to act decisively would "lead to an... unsupervised, possibly violent independence process" that could spread instability to Kosovo's Balkan neighbours, the International Crisis Group said in a report.

Kosovo has been run by the United Nations since 1999, when NATO expelled Serb forces accused of killing ethnic Albanian civilians while fighting separatist rebels.

After 18 months of talks, international mediators said there is no chance of a compromise between Serbia's offer of autonomy and the demand for independence of Kosovo's Albanian majority.

They are due to deliver a report on the latest round of negotiations to the U.N. by Monday.

Kosovo Albanians plan to declare independence in early 2008, with support from Washington and almost all EU member states that see it as the best option for stability in the Balkans.

The ICG said that once the "inevitable" failure of the mediators was reported to U.N. Secretary-General Ban Ki-moon, "France, Germany, Italy, the UK and U.S. should, despite Serbian and Russian opposition," start working on their next steps.

Russia says the issue must be decided by the U.N. Security Council. It blocked a plan for EU-supervised independence drafted by U.N. envoy Martti Ahtisaari earlier this year and wants talks to continue beyond December.

"The EU must say officially... it considers the negotiations between Serbia and Kosovo to be over, that the Ahtisaari plan is the best way forward and that it is ready to deploy field missions," the ICG said in the report.

Without a clear message from the EU, Ban could not be expected to counter Russian pressure, it said. And lacking "some semblance of U.N. cover", the bloc would be less likely to overcome divisions among member states on whether they could take up Kosovo without a Security Council Resolution.

The ICG noted that even after a peaceful transition, the issue was far from closed. Belgrade has vowed to never give up on its medieval heartland and is preparing counter-measures, which could include a trade embargo or border closures.

The West must "keep pressures and incentives on Serbia to accept reality," the ICG said, adding the prospect of EU membership might not be immediately change Belgrade's stance. (Editing by Michael Winfrey)

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