Pakistan President Pervez Musharraf "has assured us that that's, one, part of the past, and the past is the past," spokesman Scott McClellan told reporters.
Musharraf has further told the United States that "the government of Pakistan is not involved" in any clandestine nuclear sales and has launched an investigation into the matter, said McClellan.
"So obviously those are issues that we will look at as well. And we are learning more in a number of different places around the world about proliferation efforts," he said, without offering more details.
About a dozen nuclear scientists and administrators, including the "father" of Pakistan's bomb Abdul Qadeer Khan, are under investigation for allegedly selling nuclear know-how to Iran, Libya and possibly other countries in the 1980s and 1990s.
Interrogations of the scientists have sent shockwaves through Pakistan, where nuclear experts are revered as national heroes for their contributions to making Pakistan a nuclear power. All except Khan have been detained.
Pakistan's nuclear program, begun in the 1970s, was covert until May 1998 when Islamabad conducted a series of nuclear tests in reaction to rival Indias atomic detonations.