The three men - two Australians and one Yugoslav - had been working in Yugoslavia for the aid agency, Care Australia, when they were arrested. They face long prison sentences if convicted.

The trial opened as diplomatic efforts continued in Moscow to find a solution to the Kosovo conflict, although there was no sign of a breakthrough.

The Australian Ambassador, Christopher Lamb, was excluded from the court. He said his expulsion was contrary to international law.

Rageh Omaar reports: "Pratt accused of spying while posing as an aid worker"

One of the men, Steve Pratt, is charged with organising an espionage network. The others, Peter Wallace and Branko Jelen, are accused of supporting him. The charges are strenuously denied by officials of the charity.

The two Australians were arrested as they tried to leave Serbia for neighbouring Croatia.

Steve Pratt pictured with his wife

They were carrying files and computers which the Yugoslav authorities said contained sensitive information which they intended to pass to Nato.

Two correspondents from the Australian network, ABC, were briefly allowed into the court and they report that the men looked physically well.

Diplomatic drive

In Moscow, the US Deputy Secretary of State, Strobe Talbott, has again insisted on a full withdrawal of Yugoslav forces from Kosovo as a pre-condition for movement towards a political solution.

Mike Williams reports: "For almost two weeks, nothing was heard of the aid workers"

Russia again repeated its demand that Nato halt its bombing campaign to open an opportunity for a political solution.

Mr Talbott was speaking after talks with the Russian foreign minister, Igor Ivanov. He was also due to meet President Yeltsin's envoy Viktor Chernomyrdin and the Finnish president, Martti Ahtisaari.

Mr Ahtisaari, representing the EU, will accompany Mr Chernomyrdin to Belgrade on Thursday if Russia and the West agree a set of joint demands to be backed up by a resolution from the UN Security Council.

John McIntyre reports: "The decision to send more troops will send a clear message to Milosevic

Mr Chernomyrdin's previous visits to Belgrade have yielded few concrete results, with negotiations foundering on Yugoslav President Slobodan Milosevic's refusal to allow Nato countries to provide troops for any peacekeeping force in Kosovo.

However the pro-Western president of Montenegro, Serbia's junior partner in the Yugoslav Federation, Milo Djukanovic, said on Tuesday he saw "positive signals" that Belgrade might soon make concessions.

"An increasing number of people in Serbia are ... becoming aware of the pointlessness and senselessness of such a war," he said after meeting UK Prime Minister Tony Blair in London.

Extra force puts pressure on Belgrade

Nato is continuing to work on its plans to almost double the number of its troops on Yugoslavia's borders. Its reinforcement is expected to bring to 50,000 the number of its troops in the area.

Mark Laity at Nato HQ: The force will escort the refugees and then defend them

Commanders are working out the exact figure and composition of the force, which is destined for a peacekeeping role in Kosovo.

The troops will be heavily armed and ready to head into Kosovo as soon as Serb troops leave the province.

Non-Nato members will be briefed by the alliance and asked if they are interested in taking part.

Although it is not intended to fight its way into Kosovo, the peacekeeping force could provide the core of any possible ground attack.

The BBC correspondent in Brussels, Nick Childs, says the reinforcements will take the heat out of the debate over ground troops - at least for now.

Yugoslav news reports said overnight raids by Nato aircraft once again targeted the headquarters of Serbian television in Novi Sad, along with radio and television transmitters.

Sites in and around the Kosovo capital, Pristina, were also reported to have been attacked.

American defence officials said a complex used by President Milosevic on the outskirts of Belgrade was hit in earlier raids.

(Click here to see a map of latest Nato strikes)

One person was reported to have been killed and six people injured in daytime attacks across Yugoslavia on Tuesday.

Refugee influx

Orla Guerin on the Macedonian border: Macedonia is near breaking point

The need to find a solution to the Kosovo crisis has been underlined by warnings of a potential new crisis as refugees continue to arrive at the border of the Former Yugoslav Republic of Macedonia.

A United Nations report has documented allegations of widespread rapes by Serbian soldiers in Kosovo.

The report for the United Nations Population Fund (UNFPA) was based on interviews with 35 ethnic Albanian women in refugee camps in Albania.

It says the women from Djakovica, Pec and the Drenica region reported being raped sometimes for days.

They said they were taken in groups of five to 30 women and locked up in houses where the Serbian soldiers lived.

The report says women who were released had lacerations on their chest and evidence of beatings on their arms and legs. It says other rape victims were systematically killed.

• Serbs flee Yugoslavia
• History damaged by bombs
• UN accuses Serbs of ethnic cleansing
• General sent to stop troop mutiny
• Istok prison's unanswered questions

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