A Swiss court on Thursday ordered the release on bail of an alleged double agent held on suspicion of being part of a global nuclear arms smuggling network, amid allegations of US pressure in the case.

Switzerland's Federal Criminal Court said in a statement that Marco Tinner should be freed on bail of 100,000 Swiss francs (86,550 dollars, 66,300 euros), 10 times the amount paid for the release of his brother Urs Tinner last month.

It said his eight month preventive detention since the last ruling was "disproportionate".

The two brothers and their father were arrested in Germany in October 2004 and later extradited to Switzerland. The father was released in 2006.

They are accused of helping Libya develop a nuclear weapons programme and were alleged to have been in contact with Abdul Qadeer Khan, the disgraced father of Pakistan's nuclear bomb and kingpin of a network supplying nuclear technology to rogue states.

Libya publicly renounced its nuclear, chemical and biological weapons programmes in 2004.

Reports since have claimed that the family of engineers was recruited by the CIA to help thwart attempts by Libya and Iran to develop nuclear weapons, and that Swiss and US authorities colluded after they were arrested.

Early last year, the Swiss government announced it had destroyed documents on the case to stop detailed nuclear weapons plans falling into the hands of a hostile state or group, prompting fears that this might hamper the probe.

A Swiss parliamentary commission on Thursday revived a political storm by slamming that decision as "disproportionate", casting doubt on the claimed security motives and pointing directly at pressure from Washington.

The commission overseeing government affairs said in a report that the Swiss cabinet had "preferred to respond to the demands of the United States by getting rid of the whole of all the documentary evidence."

The New York Times reported in October that the real reason for the documents being destroyed was pressure from the CIA, which feared its ties with the Tinners would be exposed.

In extracts of the first interview with Urs Tinner which are due to be broadcast in a documentary on Thursday, the engineer said he helped to dismantle the Khan network, Swiss television TSR said.

However, he made no explicit reference to cooperation with the US Central Intelligence Agency (CIA), one of the documentary's producers told AFP.

Friedrich Tinner began working with Khan in the mid-1970s, using his expertise in vacuum technology to help Khan develop atomic centrifuges, according to the Times.

In 2000, the CIA is said to have recruited his son Urs, who eventually persuaded his father and younger brother to work with him as moles.

Khan has denied that Pakistan or his laboratory were linked to alleged plans found on Swiss engineers to make nuclear weapons.

Khan told Swiss newspaper NZZ am Sonntag in June that although he knew the Tinners for more than 30 years, the plans were not connected to his programme.