Officials said the Secretary of State, Madeleine Albright, has called Russian Foreign Minister Igor Ivanov to make Washington's position clear.

The warning came as Yugoslav President Slobodan Milosevic asked for Russian help in defending his country from Nato attacks.

Russian officials have indicated military aid may become available, though the State Department stressed it had no indication that Moscow was currently providing any such support.

There is a United Nations arms embargo on Yugoslavia.

On Friday Russia despatched a spy ship to the Mediterranean, where it will monitor Nato operations against Yugoslavia.

Mr Milosevic told a delegation of Russian parliamentarians in Belgrade that the Yugoslav authorities wanted military aid from Moscow to defend themselves better against the air strikes.

Sergey Baburin, leader of the Russian delegation, told a crowd in central Belgrade that Russian hearts were with them today, and he hoped that Russian missiles would be with them tomorrow.

Military aid option

The Kremlin insists that a diplomatic solution must be found for the Kosovo crisis and has remained quiet on the president's plea.

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But General Anatoly Kvashnin, Russia's armed forces Chief of Staff, revealed that military aid for Yugoslavia could be an option in certain circumstances.

General Kvashnin, quoted by Russia's Itar-Tass news agency, said the current situation did not require military supplies to be shipped into Yugoslavia.

But he added: "It is possible depending on the circumstances.

"On this we need to look at precisely what sort of aid we are talking about."

A Russian foreign ministry spokesman said that it had not received any formal request for aid.

Russian Foreign Minister Igor Ivanov says Moscow wants to see a peaceful resolution and not an escalation to the crisis.

But he has also hinted previously that the Nato air strikes automatically negated the force of UN resolutions concerning the former Yugoslavia, including an arms embargo.

"Those who have perpetrated the act of aggression themselves have drawn a line" through the resolutions, Mr Ivanov said last week.

Ship movements

Mr Ivanov has also been explaining the decision to send the Liman warship to from its Black Sea base to the Mediterranean.

He said it was "in the interest of Russian security only".

The Liman left the Black Sea port of Sevastapol on Friday and is expected to reach the Mediterranean on Saturday.

Reporters in Sevastopol said they saw no evidence that the reconnaissance ship was carrying weapons.

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At least six other vessels are scheduled to follow in the next few days - all, according to the Black Sea Fleet's commander, "combat ready".

Russian Defence Minister Igor Sergeyev told a Moscow news conference that "the aim of sending a reconnaissance ship is to obtain more specific and detailed information on the situation in the region".

Nato has expressed concern about the presence of Russian warships in the battle zone.

East-West divide

On Thursday President Yeltsin called for an emergency summit of the seven leading industrialised nations, plus Russia, to discuss the Kosovo crisis.

This was rejected by the United States. A State Department spokesman said that such a summit was not an appropriate forum to discuss Kosovo.