The US Defence Department says 660 missions were flown on Wednesday, and that the past few days have seen more sorties by Nato pilots than at any other stage of the campaign.

It says the number of missions will increase even further, with up to 800 sorties planned for Thursday.

On the 50th night of the air campaign, explosions were heard on the outskirts of Belgrade as Nato aircraft attacked what is thought to have been part of the city's mobile air defence system.

There were also further attacks on Novi Sad and Kosovo, as well as on the southern city of Nis where the Yugoslav authorities say cluster bombs again fell upon civilian areas.

(Click here to see a map of last night's Nato strikes)

But the BBC's Defence Correspondent, Mark Laity, says deteriorating weather conditions meant some of Wednesday night's planned bombing missions had to be cancelled.

According to the US Defence Secretary, William Cohen, the bombing is sapping the morale of Yugoslav forces.

US Defence Secretary, William Cohen: "The road to ruin, politically, economically and militarily"

Mr Cohen said the situation was causing army desertions and Yugoslav President Slobodan Milosevic's most senior commanders were moving their families out of Serbia.

"Milosevic may not admit that he is taking his country down the road to ruin ... but his top leadership is apparently more realistic," said Mr Cohen at a Pentagon news conference.

Mark Laity in Brussels: "Heaviest day so far"

He said the planned deployment of additional strike aircraft in Turkey and Hungary would allow Nato to intensify the pressure.

Tribute to Serbian forces

In Belgrade, President Milosevic paid tribute to the efforts of many members of Serbia's security forces who he said had been killed defending Yugoslavia in the Kosovo conflict.

The statement, read out on state-run television, gave no exact figures, but correspondents say it was a rare reference to casualties.

According to the BBC's Mike Williams in Belgrade, the statement could represent an increasing understanding on the part of Belgrade that it may be impossible to defeat the Western alliance.

President Milosevic also said that the armed forces had successfully crushed the rebel Kosovo Liberation Army.

His statement came as the army insisted it was continuing its declared pull-out of some troops from Kosovo, although Nato says it can see no evidence of troop movements.

Human rights visit

President Milosevic is due to meet the United Nations human rights commissioner, Mary Robinson, in Belgrade on Thursday.

The meeting has been criticised by some human rights campaigners who fear it will help to legitimise Mr Milosevic's authority.

Mary Robinson was visiting Nis

Mrs Robinson is in Serbia to assess the humanitarian situation in the country as a result of the fighting in Kosovo and the Nato bombing campaign.

She says she is keen to investigate the extent of casualties among innocent civilians, and the Yugoslav authorities seem eager to help.

The UNHCR head, who has been critical of the bombings, visited the southern city of Nis on Wednesday shortly after Nato attacked the city.

Serbian television reports said a number of cluster bombs had landed in the centre of the city, in an area where there are schools and houses.

Russia in crisis

Russian President Boris Yeltsin has warned that Russian attempts to broker an end to the crisis in Kosovo will stop unless they are taken more seriously.

Mr Yeltsin, speaking hours after sacking his Prime Minister, Yevgeny Primakov, and the entire cabinet, said: "Some people obviously aren't understanding our repeated proposals."

Bridget Kendall: "The US special envoy flew in to a full-blown political crisis"

Correspondents say that an end to Russian participation could destroy any chance of agreement on a UN resolution authorising an international military force in Kosovo.

Nato Secretary-General, Javier Solana, said he hoped Russia and Nato would continue negotiations and that a UN resolution on Kosovo would be ready soon.

Apology to China

China has been offered an unconditional apology by Germany's Chancellor Gerhard Schröder and Nato for the bombing of its Belgrade embassy.

But Mr Schröder, on a one-day visit to Beijing, said China was resolute in its demand that Nato halt air strikes on Yugoslavia before it would support a peace deal.

His visit came as the remains of the three people killed in the embassy attack were returned to China.

• Protests delay Green's kosovo debate
• Demonstrators march on US embassy
• The costs of war and peace
• Fifty days on - how credible is Nato
• Pilots playing 'cat and mouse'

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