The foreign ministry in Moscow accused the Kosovo-Albanian militia of breaking United Nations resolutions with impunity.

It accused the KLA of threatening the Russian peacekeepers and of carrying out attacks against Serbs.

The robust statement was prompted by comments by the KLA's political leader, Hashim Thaci, on Saturday, after the brief detention of one of its senior militia commanders.

BBC's Orla Guerin: Nato is doing its best to control the tension

Mr Thaci said his detention by Russian peacekeepers was a premeditated political act and called for the Russian troops to be brought under Nato control.

War of words

In a growing war of words, Moscow has said his comments amounted to public and unacceptable threats against the peacekeepers.

"This behaviour of KLA leaders is absolutely unacceptable and impermissible, and cannot be called anything other than a deep provocation", said the foreign ministry.

Church lightly damaged

"Urgent and effective measures are needed to oblige the KLA to unconditionally meet all terms of the peacekeeping process in Kosovo and not permit the further course of events toward creating a threat to all peacekeeping efforts in Kosovo."

When the Russian contribution to K-For was first announced there were concerns that they would be seen as proxies for the Serbian forces, leading to friction with the KLA.

There seems little doubt of the bad feeling between the Russian forces and most of the Kosovo-Albanian population.

There are some 3,600 Russian peacekeepers in Kosovo. They operate as part of K-For in sectors controlled by Nato states, but they are not directly under Nato's command.

KLA accused of Serb violence

In the past Moscow has described the KLA as a terrorist organisation.

Now, the Russians have also accused the militia of carrying out violent attacks on Kosovo Serbs and behaving as if it controls the territory.

In the latest attack against Serbs, a grenade was thrown at the main Serbian Orthodox church in the Kosovan capital, Pristina.

The explosion caused some light damage to the building, but no casualties.

Serb leaders said the bombing was part of a systematic campaign to drive them out of Kosovo.

The United Nations special representative in Kosovo, Bernard Kouchner, condemned the attack as absolutely unacceptable.