Mr Yeltsin branded the action a "gross mistake" and said that the US would be held to account for the air strikes.

But he said that good sense had prevailed and his country was resisting the temptation to use the extreme measures. He did not specify what the measures were.

"Nato's aggression against Yugoslavia is a gross mistake by American diplomacy and Clinton, and they at the end will be held to account for it," Mr Yeltsin said in a statement reported on the Interfax news agency.

The BBC's Bridget Kendall: "In Moscow, protests degenerated into egg throwing"

The first report from the agency said Mr Yeltsin said Russia had prepared a series of extreme measures, but had decided against using them yet.

The statement was later clarified to report that Mr Yeltsin said a number of options were open to Russia.

The Russian government, parliament, officials and people are angry at Nato's use of force against Serbia and the action will undoubtedly affect Russia's relations with the West.

Threat to weapons deal

Robert Parsons: Russia and the Russians are very angry

The Itar-Tass agency reported that President Yeltsin also warned that US-Russian nuclear disarmament programmes had been put in doubt because of the strikes.

The warning apparently came in a long telephone conversation with US President Bill Clinton on Wednesday.

Tass also reported that President Boris Yeltsin has assigned the Russian Foreign Ministry to work for convening a conference of foreign ministers of the six-nation Contact Group on Yugolsavia, which includes western countries and Russia.

A political price

"The conference is expected to discuss ways of a peaceful solution to the Kosovo problem," Russian Foreign Minister Igor Ivanov told journalists.

He was speaking after a meeting with Mr Yeltsin on Thursday morning.

BBC Moscow correspondent Robert Parsons says Russia will keep calling for meetings of the UN Security Council.