The United States is calling Friday's release from house arrest of Pakistani nuclear scientist A.Q. Khan "extremely regrettable."  The State Department says Khan, who admitted sharing Pakistani nuclear secrets with Libya, Iran and North Korea, remains a serious proliferation risk.

Officials say the United States has not been able to get official confirmation of the decision by the Pakistani High Court in Islamabad. But they say there is no reason to doubt reports that his house arrest has been lifted, and are making clear U.S. displeasure.

At a news briefing, State Department deputy spokesman Gordon Duguid said the apparent release is extremely regrettable.

"On the general principle of A.Q. Khan and his status, let me say that in our view it would be unfortunate if the court released him," Duguid says. "We believe A.Q. 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."

Secretary of State Hillary Clinton also discussed the release briefly at a photo session with Philippine President Gloria Macapagal-Arroyo, saying the Obama administration is "very much concerned" about it and would have more to say later.

Duguid said the Pakistani government is "well aware" of the U.S. position on the issue and said the U.S. Embassy in Islamabad intends to pursue the matter with Pakistan authorities.

News reports say the Pakistani court also lifted a ban on contacts with the news media by Khan, which he in any case had largely ignored in recent months.

Khan, considered the father of Pakistan's nuclear bomb program, told reporters outside his home in the capital that he would not cooperate with any foreign investigations of his proliferation network.

The United States has been rebuffed in efforts to interview him.

Spokesman Duguid said despite the release, U.S. and allied efforts to, in his words, roll up the Khan network will continue unabated. Last month, the United States imposed financial sanctions on Khan and 12 of his associates as well as three companies linked to the operation.

A senior official who spoke to reporters here said the Obama administration's new special representative for Afghanistan and Pakistan Richard Holbrooke is likely to raise the case when he visits Islamabad next week.

The official said while Holbrooke's portfolio centers on the Afghan conflict and related activity along the Pakistani border with that country, he retains the flexibility to discuss all issues on the bilateral agenda.