The United States on Monday unveiled sanctions against detained Pakistani scientist Abdul Qadeer Khan, 12 associates and three firms linked to his nuclear proliferation network.

The sanctions forbid the 16 from having business dealings with the US government or private US firms in what the State Department says is a renewed bid to make sure the network has been shut down entirely.

"This is a prudent effort on our part," a State Department official told AFP on the condition of anonymity.

The official said that Washington has had concerns that elements of the network could still be active since Khan, the father of Pakistan's atomic bomb, acknowledged his role nearly five years ago.

Khan has been effectively under house arrest in Islamabad since February 2004, when he confessed on television to sending nuclear secrets to Iran, Libya and North Korea, although he retracted his remarks later.

"We want to close any loopholes. We think that this network has been dismantled, but we don't know that with absolute certainly," the US official said.

The sanctions are also a signal to other countries.

"Certainly we hope it will encourage other countries to frankly go out there and look at their existing sanctions and see if they can tighten them," the official said.

In June last year, former UN arms inspector David Albright urged the US government to pressure Pakistan to allow US or UN experts to question Khan over the sale of nuclear know-how to Iran or North Korea.

It was important to obtain such information in case Khan, a hero for many in Pakistan, is released, he added.

In Islamabad, Pakistani Prime Minister Yousuf Raza Gilani said Monday: "AQ Khan is a closed chapter. Pakistan is mindful of its responsibilities as a nuclear weapon state as well as its international obligations."

"The sanctions are not against the state or government. The US has in fact applauded Pakistan's contributions to promote global non-proliferation," the prime minister's office quoted him as saying.

Howard Berman, who chairs the US House Committee on Foreign Affairs, welcomed the "belated" sanctions, but said more work is needed to fight proliferation as Barack Obama prepares to assume the presidency next week.

"Equipment and technology from this network may still be circulating, and new suppliers could well spring up to take Khan's place," Berman, a Democrat, said in a statement.

Most of those sanctioned have for years been mentioned in the media over their links to Khan: Turkish businessman Selim Alguadis and his firm EKA Elektronik Kontrol Aletleri Sanayi ve Ticaret AS, Pakistani scientists Muhammad Farooq and Muhammad Nasim ud Din, Sri Lankan scientist Buhary Seyyed Abu Tahor, German engineers Gerhard Wisser and Gotthard Lerch, Swiss engineer Daniel Geiges and British businessmen and brothers Paul and Peter Griffin.

Set up by Wissar, the South African firm Tradefin Engineering is also on the list which nevertheless does not say whether these individuals or firms have business interests in the United States or retain contacts with US firms.

Official say the measures ban credit, guarantees, or insurance in support of US exports to the sanctioned individuals; forbid US imports from the sanctioned entity; and freeze the assets of a sanctioned entity within US jurisdiction.

The statement mentions other people without listing their nationalities or links to Khan: Kursad Zafer Cire, Shamsul Bahrin bin Rubikan, Shah Hakim Shahnazim Zain and the Turkish firm ETI Elektroteknik Sanayi ve Tikaret AS.

The State Department said many of Khan's associates are either in custody, being prosecuted, or have been convicted of crimes, but did not give details on the status of each.