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Nato has begun its first wave of strikes against Serbian military targets.

US defence officials said the attacks began with air and sea-launched cruise missiles.

The many threats against President Milosevic were finally acted on with explosions first heard at about 1900 GMT.

BBC Correspondents say two explosions have been seen on the outskirts of Belgrade.

An explosion was seen near a big Serb military base near Belgrade.

Jacky Rowland in Pristina at 1925GMT: "It's dark. It's silent. Everyone feels that they have lost control of their lives."

Detonations were also heard in the capital of Kosovo, Pristina. Residents said at least four huge explosions were heard - one quite loud - and air raid sirens sounded.

Pristina was plunged into darkness after the explosions.

The missoles are expected to pave the way for huge bombing raids on Serb forces by up to 400 Nato warplanes on standby in the region.

Air raid sirens were also heard in Belgrade.

The beginning of the operation was announced by the Nato Secretary-General.

Waves of Nato planes took off from the an airbase at Aviano in northeastern Italy shortly after nightfall.

Hours earlier, eight American B-52 bombers, armed with cruise missiles, left from an airbase in England.

President Clinton: "Our Nato allies unanimously support this action"

President Bill Clinton said the aim of the action was threefold: to demonstrate Nato's opposition against agression, to deter President Slobodan Milosevic from further attacks and to diminish the Serbs' military capability.

''Kosovo's crisis is now full blown,'' he said. ''If we don't act clearly it will get even worse. Only firmness now can prevent greater catastrophe later.''

Military observers say the warplanes will meet a stiffer challenge than air missions over Iraq; western leaders have acknowledged they risk allied casualties.

This is the first time in the alliance's 50-year history that it has attacked a sovereign European nation.

Nato Secretary-General Javier Solana gave the go-ahead for air strikes against Yugoslavia on Tuesday night, saying there was no option if Nato is to avert a humanitarian catastrophe in Kosovo.

Thousands of Kosovo Albanians have been fleeing the latest Serb-led offensive heading south into the former Yugoslav republic of Macedonia.

Jeremy Cooke in Belgrade sees "a bright orange flare tracking across the sky"

Yugoslavia was placed on high alert after the government declared a state of emergency. Military preparations were being made with air-raid shelters and bunkers prepared for civilians.

The United Nations Security Council is to meet shortly at Russia's request.

Defiant Serbs

On Wednesday afternoon, Serb leader Slobodan Milosevic called on his people to be prepared to defend their country "by all means".

In a nationally televised address he warned them that "at stake is the freedom of the entire country".

"We shall defend the country if it is attacked," he said.

The BBC's Defence Correspondent, Mark Laity, says Nato is fully aware of the capability of Serb air defences and it will be a tougher campaign than those waged by the British and Americans against Iraq.

Blaming Yugoslav Government "intransigence", the Nato secretary-general has said Nato's quarrel was not with the Yugoslav people and that the attacks would be aimed at weakening the Yugoslav army and special police forces.

President Boris Yeltsin appealed to world leaders in a national television address on Wednesday to keep Bill Clinton from making what he described as a "terrifying and tragic" mistake of launching air strikes against Serbia. Russia says it is ready to call an emergency meeting of the Security Council.

Its Prime Minister, Yevgeny Primakov has warned that Russia's relations with Washington will be damaged and European stability harmed.