WASHINGTON — The United States on Monday slapped sanctions on people and companies connected to the black market nuclear network led by Pakistani scientist Abdul Qadeer Khan.

U.S. officials called the sanctions "a warning to other would-be proliferators" and urged the world to "remain vigilant" to make sure Khan's associates are stopped from leaking sensitive nuclear information or equipment in the future.

The State and Treasury Departments said the United States had placed sanctions on 13 people and three private companies for their involvement in Khan's network, which leaked atomic secrets to Iran, Libya and North Korea.

Those hit with sanctions include British, German, Turkish, Swiss and Sri Lankan nationals. Any bank accounts or other financial assets found in the United States that belong to those designated will be frozen. Americans also are prohibited from doing business with them.

Khan, the architect of Pakistan's atomic weapons program, confessed in 2004 to playing a major role in the international nuclear smuggling ring.

"With the assistance of Khan's network, countries could leapfrog the slow, incremental stages of other nuclear weapons development programs," the State Department said in a statement. "The network's actions have irrevocably changed the proliferation landscape and have had lasting implications for international security."

Khan has been kept under de facto house arrest in the Pakistani capital since his 2004 confession. He was quickly pardoned at the time by then-President Pervez Musharraf. The revelation spared Pakistan even greater international condemnation over the leaking of nuclear technology to Iran, Libya and North Korea, which, at the time, were all at loggerheads with the West.

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Recently, however, Khan has disowned his confession in media interviews and said the army had known all about at least one act of proliferation in 2000.

Rep. Howard Berman, D-Calif., called the sanctions belated but welcome; he warned that "equipment and technology from this network may still be circulating, and new suppliers could well spring up to take Khan's place."

Berman, chairman of the House Foreign Affairs Committee, urged Obama to redouble U.S. efforts against nuclear black markets and called on Congress to be ready to provide the money and legal authority that Obama "may need to end the activities of these merchants of mass destruction."

One of those hit with U.S. sanctions, German engineer Gotthard Lerch, was sentenced last year to 5 1/2 years in prison for his role in the Khan network. He was convicted by a German court of breaking export and weapons laws by sending uranium-enriching equipment to Libya between 1999 and 2003 despite knowing the North African nation was trying to build nuclear weapons.

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On the Net:

Treasury list of those designated: http://www.treas.gov/offices/enforcement/ofac/actions/20090112.shtml

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AP Economics Writer Jeannine Aversa contributed to this story.