The arrest of two top Pakistani nuclear scientists in Islamabad has been confirmed by Pakistani sources in London, who say the men have been accused of passing on nuclear secrets to Iran.

Dr Farooq Mohammed and Dr Yasin Chohan were key members of the team responsible for Pakistan's 1998 nuclear tests.

They are described as director and laboratory director respectively of Pakistan's secret uranium enrichment facilities at Kahuta, situated between Rawalpindi and Islamabad.

The two scientists are the latest in a list of nuclear experts detained at the behest of the US.

Two years ago under US pressure the Pakistani authorities arrested Dr Sultan Bashiruddin Mehmood, who had designed the Khusab nuclear power station, and subsequently offered his services to the Taliban regime in Kabul.

One of his colleagues who worked with him in Kabul was shipped out to the Pakistan embassy in Myanmar before US counterintelligence and terrorism experts had a chance of getting to him.

Mohammed and Chohan have worked under the  'father' of the Pakistani nuclear programme, Bhopal-born Dr Abdul Qadeer Khan.

Khan did much of his early research at a uranium research centre in Almelo, Holland, jointly owned by a three-nation European consortium, before he returned to Pakistan.

The uranium enrichment plant subsequently set up in Kahuta, also known as the AQ Khan research laboratories, drew heavily on the expertise that Khan acquired in Almelo.

A question mark rises over Khan now that such senior members of his team have been arrested. In recent months Khan himself has been under suspicion over allegations that he transferred nuclear secrets to North Korea as well as earlier to Iran.

According to the Pakistani sources two FBI members were part of the team that detained Mohammed and Chohan in Islamabad on Wednesday. Mohammed had just returned home after attending a relative's funeral.

The Pakistani authorities have yet to confirm the arrests, but opposition senators from the country's upper house of parliament have lodged a protest, saying the arrests are a grave threat to national security.

The Pakistani foreign ministry said in a statement that Islamabad has an uncompromising policy not to transfer nuclear technologies to third countries.

A spokesman for the ministry refused to comment on why the US has imposed sanctions on the Kahuta laboratories after accusing its scientists of providing material support to a country or people trying to develop weapons of mass destruction or the missiles that carry them.