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

The many threats against President Milosevic were finally acted on at 1900 GMT when US warplanes and warships began firing missiles at key air defence sites.

BBC Correspondent in Belgrade, Jeremy Cooke, says journalists in Belgrade have witnessed flashes on the horizon outside the capital. A military airfield lies approximately 15 km from the hotel where they are staying.

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

This is 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 have been sounding in Pristina.

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

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

President Slobodan Milosevic has promised to resist the offensive by "all means".

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.

Yugoslavia was placed on 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.