(Adds letter by Britain, France, Germany and Italy)

By Mark John and David Brunnstrom

BRUSSELS, Dec 7 (Reuters) - NATO nations pledged on Friday to provide enough troops to put down any violence as Kosovo heads towards a declaration of independence from Serbia, expected within weeks.

Britain, France, Germany and Italy urged fellow EU states in a letter to accept that negotiations on Kosovo's future had been exhausted and that the time had come to settle its status -- without United Nations backing, if necessary.

In a separate move that drew immediate fire from Russia, NATO countries agreed that their 16,000 KFOR peacekeepers could stay in Kosovo on the basis of their existing U.N. mandate, even after independence.

"KFOR shall remain in Kosovo on the basis of U.N. Security Council resolution 1244, unless the Security Council decides otherwise," NATO ministers said in a final communique.

"We renew our commitment to maintain KFOR's national force, contributions, including reserves, at current levels and with no new caveats," they added, using the military term for limits that nations sometimes impose on what their troops can do.

Such caveats meant NATO was caught badly off guard during rioting in north Kosovo in 2004, which it struggled to control. The alliance has up to four reserve battalions -- each with several hundred troops -- on standby for trouble.

Washington and most EU states are likely to recognise a declaration of independence by Kosovo and are confident its leaders will wait until around late January to enable NATO and the European Union to prepare for it.

"There is still a lot of work to do to make sure we have full commitment to the principles embodied in the Ahtisaari plan," U.S. Secretary of State Condoleezza Rice said of a U.N. blueprint for independence by special envoy Martti Ahtisaari, which has security provisions for Kosovo's Serbs.

The vast majority of EU member states are now seen as ready to accept Kosovo's independence but Belgian Foreign Minister Karel De Gucht, who hosted a dinner for EU and NATO counterparts on the eve of the meeting, said full consensus was elusive.

"If we do not get consensus, we are not in the driving seat," he told reporters.

A "troika" of U.S., Russian and European mediators failed to secure agreement between Pristina and Belgrade over Kosovo. A Western diplomat said they would deliver their report to the United Nations on Friday, before a Dec. 10 deadline for a deal.

The envoy told Reuters the report prescribed no way forward on the fate of the breakaway province, reflecting differences between the West and Serb ally Russia.

"SLIPPERY SLOPE"

The agreement that U.N. Security Council resolution 1244 can justify NATO's presence in Kosovo even after independence is crucial, as several nations such as Germany had harboured doubts over whether it could continue to apply.

U.N. Security Council veto-holder Russia has not made clear whether it will challenge such an application of the resolution. But Foreign Minister Sergei Lavrov slammed it as potentially undermining basic standards of international law.

"Anybody who goes in contravention to those is on a very slippery downward slope and it certainly won't help the rest of us in Europe," he said after brief talks with NATO counterparts, referring to concerns it could encourage other separatist moves.

British Foreign Secretary David Miliband urged European nations, which are due to take over policing in Kosovo from the United Nations, to prevent a repeat of the violence seen in the Balkans in the wars of the 1990s.

"This is in Europe's backyard and European nations need to show real leadership ... we know from the mid-1990s the cost of Europe wringing its hands and failing to provide leadership."

A Western official familiar with the letter sent by Britain, France, Germany and Italy to other EU states on Friday said the four had suggested the EU should offer more help to Serbia in its goal of being named a candidate for membership of the bloc.

Kosovo has been administered by the United Nations since a 1999 NATO bombing campaign to halt killings and ethnic cleansing by Serb forces against the 90 percent ethnic Albanian majority in the province, which Belgrade says must remain under its sovereignty. (Additional reporting by Matt Robinson in Pristina and Sue Pleming in Brussels; writing by Mark John; editing by Andrew Roche)

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