President Barack Obama wants assurances from Islamabad that released Pakistani scientist Abdul Qader Khan, accused of leaking atomic secrets, isn't involved in any of the activity that led to his arrest.
"Obviously the president and this government want assurances that Dr Khan is not engaged or involved in any of the activity that resulted in his house arrest earlier," White House spokesman Robert Gibbs said on Friday.
A Pakistani court Friday ordered the release of Khan, considered the father of Pakistan's atomic bomb, who spent five years under house arrest after admitting selling nuclear secrets to Iran, North Korea and Libya.
"We've seen the reports of the release but have yet to receive official word from the government," Gibbs said. "Obviously, this president has made clear many times the great concern that he has about nuclear proliferation."
Secretary of State Hillary Rodham Clinton also expressed concern about the release of Khan. In the House, Foreign Affairs Committee chairman Howard Berman suggested that US aid to Pakistan could suffer because of the development. Berman said he is "deeply concerned" that Pakistan may be giving Khan "license to resume, perhaps directly, his past actions to aid, abet and profit from the spread of nuclear weapons."
Congress, Berman said, will take Pakistan's refusal to allow US officials to interview Khan "into account as we review and create legislation on US-Pakistan relations and the circumstances under which US assistance is provided to Islamabad."
CIA spokesman George Little called Khan "one of the most dangerous proliferators in history" and praised the disruption of his smuggling network as "a genuine intelligence success."
Earlier, State Department spokesman Gordon Duguid describing Khan as "a serious proliferation risk" said in US view "it would be unfortunate if the court released him."
"We believe AQ Khan remains a serious proliferation risk. The proliferation support that Khan and his associates provided to Iran and North Korea has had a harmful impact on international security, and will for years to come."
"The Pakistanis are well aware of our position on this," he said. "This is not a new position."
Asked if the US was anticipating Khan's release when it recently announced sanctions targeting him, the spokesman said: "The sanctions were announced to target the A.Q. Khan network, which we are actively pursuing and we are trying to roll up."
"And the United States and its international partners have done a good job in tracking down this network. That should have no effect or influence on whether or not Mr Khan is let out of his current status of detention," he said.