Australian former army major, Steve Pratt, 49, said he had used his position as a worker for the international aid organisation, Care, to gather intelligence about Kosovo and the effects of bombing.

But Foreign Minister Alexander Downer said the espionage claims were "absolutely preposterous".

Australia is also calling for the immediate release of Mr Pratt's colleague, Peter Wallace, who was also detained by the Yugoslav authorities on 31 March as the pair left Belgrade for Montenegro to help refugees.

Mr Downer summoned the Yugoslavian Ambassador in Australia, Dragan Dragojlovic, to demand immediate access to the two men.

Concern over confession

Before the meeting, Mr Dragojlovic told reporters: "I cannot force my government to release them because if they did something against law, I cannot tell my government, yes, you must release them."

He denied that Mr Pratt had been coerced into admitting he was a spy and said that Nato's bombing would have to stop before the men could be freed.

Steve Pratt: No physical signs of ill-treatment

Mr Dragojlovic added: "I am sure that nobody and no-one tried to force Mr Pratt or Mr Wallace to admit anything."

But Care Australia's emergency co-ordinator, Brian Doolan, said the spying allegations were "absolute lunacy".

He said: "Steve is not a spy. He has gone on television in Serbia and made a statement, I think, clearly under some form of duress or some form of threat."

Former Prime Minister Malcolm Fraser, the chairman of Care Australia, is flying to Moscow on Monday night, hoping to persuade Russian Prime Minister Yevgeny Primakov and other high-level Russian officials to pressure Yugoslavia to release the men.

Russia is one of the few nations to support Yugoslavia and condemn the NATO bombing, although it has stopped short of offering military assistance.

No sign of second detainee

Serbian television said Mr Pratt was part of a network of agents which had been smashed by the Yugoslav authorities.

Mr Pratt bore no obvious signs of physical mistreatment and spoke calmly and clearly in English, with his words translated into Serbian.

Raja Jarrah, Care International UK: No reason to believe the two involved in anything illegal

He was shown in profile and the television referred to him as "major".

"When I came to Yugoslavia, I performed some intelligence tasks in this country by using the cover of Care Australia," Mr Pratt said.

"My concentration was on Kosovo and some effects of the bombing. I misused my Yugoslavian citizen staff for the acquisition of information.

"I realise that the damage was done this country by these actions, for which I am greatly sorry. I always did and still do condemn the bombing of this country."

There was no mention of Mr Wallace in the television report.