The founder of Pakistan's nuclear program, Abdul Qadeer Khan, (2008 file photo)

State Department officials believe the A.Q. Khan network is no longer operating, and say the sanctions are in part intended to serve notice on would-be proliferators that such activity will be detected and punished.

An announcement said U.S. financial sanctions have been imposed against three companies and 13 individuals, including British, German, Turkish, Swiss and Sri Lankan nationals. Among them is a German engineer imprisoned last year for brokering the shipment of uranium-enrichment equipment from Pakistan to Libya before Libya scrapped its nuclear program in 2003.

The action freezes any assets the firms and persons may have in U.S. financial institutions, and bars U.S. citizens and companies from doing business with them.

A.Q. Khan, considered the architect of Pakistan's nuclear weapons program admitted transferring nuclear technology and hardware to other countries in 2004. Kahn, who is considered a national hero in Pakistan, was pardoned by former Pakistan President Pervez Musharraf but remains under de facto house arrest in Islamabad.

State Department Spokesman Gordon Duguid said while many of A.Q. Khan's associates have either been convicted of crimes or are being prosecuted, it should not be a cause of complacency about the proliferation threat:

"We believe quite the opposite. We believe these sanctions will prevent future proliferation activities by these private entities, provide a warning to other would-be proliferators, and demonstrate our ongoing commitment to using all available tools to address proliferation-related activities. So this is on-going work. This is by no means an end to anything. Proliferators are out there. They will continue. We will continue to work against them," he said.

A general view shows the reactor building of the Bushehr nuclear power plant in southern Iran (File)

U.S. officials said the A.Q. Khan ring provided Iran and Libya with designs, and components of uranium-enrichment centrifuges and in some cases complete units, and that it is believed to have provided similar technology to North Korea.

A written statement from the State Department said the break-up of the Pakistani-based operation "irrevocably changed" the proliferation landscape. It cited unprecedented international cooperation against nuclear smuggling, including the adoption in 2004 of U.N. Security Council Resolution 1540, making the fight against such activity by countries and non-state actors a binding obligation for U.N. members.