By Paul Anderson BBC correspondent in Islamabad

Nuclear-capable Hatf missiles on parade in Islamabad

The arrests have been linked to suspicions in the West that Pakistani scientists may have been transferring nuclear know-how to Iran.

Pakistani official deny this, and have not confirmed the scientists' arrest.

Newspapers named the arrested men as Dr Farooq Mohammed and Dr Yasin Chohan - both key players in 1998's successful bomb tests, says the opposition.

Dr Mohammad and Dr Chohan are the director and laboratory director respectively of Pakistan's uranium enrichment facility, the Kahuta Research Laboratories.

Opposition members of the country's upper house of parliament said the two scientists were detained at the behest of the FBI around two weeks ago.

The senators walked out of a session in protest, saying the arrests were a grave threat to national security.

US sanctions

A subsequent Foreign Ministry statement, while failing to confirm the arrests, did say that individuals who work on sensitive programmes, particularly in nuclear states, are governed by a stringent dependability and debriefing programme.

The matter of the arrests, the statement said, fell within that programme.

On the allegation the scientists may have been transferring nuclear know-how to Iran, the statement said Pakistan had an uncompromising policy not to transfer nuclear technologies to third countries.

Western media reports citing unnamed intelligence officials have alleged that Pakistan has helped the nuclear weapons programmes of North Korea and of Iran.

It has just agreed to snap inspections of its nuclear power facilities, demanded because of fears that Teheran is trying to develop nuclear weapons.

Earlier this year the Americans imposed sanctions on the Kahuta Research Laboratories, saying the establishment was providing material support to a country or people trying to develop weapons of mass destruction or the missiles that carry them.

The country wasn't named and the allegation was denied in Pakistan.