Pakistan says it is investigating whether its scientists may have spread sensitive nuclear technology abroad.
Pakistan says it needs nuclear weapons because India has them
Islamabad has denied ever helping states such as Iran and North Korea with their nuclear programmes.
But the government conceded for the first time on Monday that certain scientists may have been acting independently of the authorities.
Amongst the scientists being questioned is Abdul Qadeer Khan, the so-called father of Pakistan's nuclear bomb.
Mr Khan was being questioned in connection with "debriefings" of several other scientists working at his Khan Research Laboratories, a uranium enrichment plant near Islamabad, Foreign Ministry spokesman Masood Khan said on Monday.
"He is too eminent a scientist to undergo a normal
debriefing session," the spokesman told Reuters news agency. "However, some questions have been raised with him in relation to the ongoing debriefing sessions."
Mr Khan denied reports that the scientist, a national hero for his work on Pakistan's bomb, was under restriction.
Earlier on Monday, Information Minister Sheikh Ahmed Rashid said certain scientists were being questioned about possible involvement in the proliferation of sensitive technology.
"Some individuals may have been doing something on their own. We are investigating," he told the Associated Press news agency.
At least two other scientists from the Khan Research Laboratories, Pakistan's top nuclear laboratory, have been taken for questioning this month.
One of the scientists has been allowed to go home, but the authorities are still holding Mohammad Farooq, the director general at Khan Research Laboratories, until recently called the Kahuta Laboratories.
Abdul Qadeer Khan - a national hero in Pakistan
Dr Farooq, an aide to Abdul Qadeer Khan, has been undergoing "a dependability and debriefing session", the authorities say.
He has long been an aide to Abdul Qadeer Khan.
Pakistan has consistently denied having a policy to export nuclear expertise to any other country.
But allegations that nuclear skills and know-how have been transferred to countries dubbed "rogue states" by Washington refuse to go away.
Over the weekend, reports in the New York Times and the Washington Post fuelled speculation that Pakistan may have provided Iran and North Korea with crucial technology to enrich uranium for nuclear weapons use.
The government of Pakistan, which first tested nuclear weapons in 1998, denies the allegations point blank.
It says it takes its responsibilities as a nuclear state very seriously, and has not authorised any transfers of sensitive nuclear technology to other countries.
Mr Rashid said the decision to look into the allegations had been taken after the International Atomic Energy Agency voiced concern.