Possible revival of the AQ Khan run nuclear-smuggling network is an area of ongoing concern for Washington even as Islamabad claims the notorious Pakistani scientist is out of business.
"Obviously, the AQ Khan network has - or is responsible for some of the most serious cases of proliferation in recent decades," State Department spokesman Philip Crowley told reporters on Tuesday when asked to comment on an article in the Washington Times.
"We have worked intensively with Pakistan and other countries to shut down that network," he said.
"And while AQ Khan himself is out of business, according to the government of Pakistan, we watch his network very closely for signs that others within his realm are still in business."
"It is something - it's an ongoing focus of ours, and because we - this is a part of our broad international effort to try to stem sources of proliferation around the world. So it is an area of ongoing concern," Crowley said.
The Times reported last week that scientists, engineers and financiers involved in the AQ Khan nuclear-smuggling network are being contacted by several governments in an effort to lure these specialists out of retirement.
The development is raising concerns among US intelligence agencies about the revival of the proliferation network that was thought to have been shut down years ago, it said.