Foreign Ministry spokesman Masud Khan said Pakistan was determined to get to the bottom of allegations that nuclear technology might have been transferred to Iran.
He said it began questioning scientists from a state-run laboratory, set up by the father of its bomb programme, Abd al-Qadir Khan, five to six weeks ago after approaches by the
UN's International Atomic Energy Agency (IAEA) and information from the Iranian government that "pointed to certain individuals".
There are indications that certain individuals might have been motivated by personal ambition or greed. But we have not made a final determination," he said.
He stressed that the government itself had never been involved in nuclear proliferation. "It takes its responsibility as a nuclear weapons state very seriously," he said.
"The government of Pakistan has not authorised or initiated any transfers of sensitive nuclear technology or information to other countries," he said. "This is out of the question."
Musharraf assured US his govt
had no nuclear links to Iran
The spokesman said anyone involved in any nuclear technology transfers would be punished: "Nobody is above the law."
On Monday, Islamabad revealed that AQ Khan, revered as a national hero for developing a nuclear bomb tested in 1998 to match that of rival India, was being questioned in connection with "debriefings" of several scientists working at his Khan Research Laboratories, a uranium enrichment plant near Islamabad.
The admission came after diplomats said last month the IAEA was probing a possible link between Iran and Pakistan.
This followed Tehran's acknowledgement that it had used centrifuge designs that appeared identical to ones used in Islambad's quest for the bomb.
Tehran, accused by Washington of seeking to develop nuclear arms, told the IAEA it had obtained the designs from a "middleman", a Western diplomat said at the time.
On Sunday, Islamabad said Yasin Chohan, one of three Pakistani scientists detained earlier in the month, had been allowed home after a "personnel dependability and debriefing session".
It said two others, Muhammad Faruq and another identified only as Said, were "still undergoing debriefing".
On Monday, Bush administration officials said Pakistani President Pervez Musharraf had assured Washington that his government had not - at least "in the present time" - provided any nuclear secrets to countries like Iran and North Korea.
White House spokesman Scott McClellan called Musharraf's personal assurances "important" and added that close cooperation between the United States and Pakistan in the so-called "war on terrorism" would continue.
Past allegations of Pakistani technology transfers, not only to Iran but also to North Korea, have been an embarrassment for the White House, which relies on Pakistan as a key ally in its battle against al-Qaida.