The allegations, which are under investigation in Pakistan, could damage a growing U.S. relationship with Pakistan centered on countering terrorism.
Asked whether technology was shipped to any of the three countries before or during President Pervez Musharraf's (search) tenure, Ambassador Ashraf Qazi said that "as far as we know, none was shipped out -- ever. Nobody has presented us with evidence that this happened at such and such a time."
"We have investigated and we haven't come across any evidence," he said after a speech at the Middle East Institute, a private think tank. In the speech, Qazi expressed hope that India and Pakistan could resolve their disagreement over the contested Kashmir territory.
Qazi said that based on information provided by the International Atomic Energy Agency (search), "We are investigating scientists to see if something happened without the knowledge of the government."
The ambassador did not say what Pakistan had been told by the U.N. agency.
Libya announced last month it was giving up its nuclear, biological and chemical weapons programs and pledged to name its suppliers.
U.S. officials say many of the names probably will be Pakistani. They say evidence points to Pakistani nuclear experts as the source of at least some technology that Libya used; similar reports have arisen about probable Pakistani assistance to Iran and North Korea.
While strongly denying government involvement, Pakistani authorities last month detained two top nuclear scientists and questioned the father of Pakistan's nuclear program, Abdul Qadeer Khan. Pakistani officials said they were acting on information from Iran and the International Atomic Energy Agency.
"No one is too big" not to be questioned, Qazi said when asked about Khan.
In December, Pakistan's government said it was questioning a number of its nuclear scientists on suspicion that "ambition and greed" may have led them to sell their knowledge to Iran. Islamabad denied government involvement in the plot and said any leaks were limited to Iran.