The Pakistan government is taking the issue of nuclear proliferation "very seriously", US State Department spokesperson Richard Boucher said on Wednesday.

Asked whether he finds credible the statement by Pakistani scientist Abdul Qadeer Khan taking responsibility for nuclear proliferation, Boucher said, "I don't. The Pakistani government is investigating this."

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The US is "following this closely. We will see what they decide to do at this point", he added.

Boucher said the process of investigation undertaken by Pakistan demonstrates that President Pervez Musharraf and his government take seriously their commitments, their assurances and "they are not going to allow their technology to be used to help other nations".

For years, he said, a major focus of the non-proliferation efforts has been on private sales, transfer of equipment from companies and transfer of expertise. "We have had massive programmes in the former Soviet Union to try to make sure that scientists there that had nuclear expertise were not recruited and did not go elsewhere."

Meanwhile, White House spokesperson Scott McClellan said the US would work closely with Pakistan to win the war on terrorism.

"We appreciate their efforts to address what is a serious concern, which is proliferation of weapons of mass destruction."