Disgraced Pakistani scientist A Q Khan's clandestine nuclear proliferation network is still functioning despite Islamabad's claims that it has been dismantled, a United States think tank has said.

It also accused Pakistan of interfering with the investigators' inquiries.

"US officials claim that the Khan network has been dismantled and the Pakistani government says the case is closed, but according to a testimony before the House Subcommittee on International Terrorism and Non-proliferation, that is not the case," New York-based Council on Foreign Relations noted in a newsletter.

The CFR's Daily Analysis quotes Leonard Weiss, an independent nonproliferation expert, telling Congress that 'at least some parts of the network are definitely still functioning'.

"The story of A Q Khan underscores the importance of nonproliferation efforts in an era when technology and expanding trade increasingly favour smugglers.

"Such a challenge requires a creative solution, and the Proliferation Security Initiative a project of the Bush Administration, may hold some of the answers. An interactive guide from MSNBC describes how renegade nations may go about concealing their own nuclear programs," Eben Kaplan said in a newsletter published Thursday.

Institute for Science and International Security President David Albright has testified that the UN nuclear watchdog IAEA officials have not been able to question Khan directly and that 'key questions remain unanswered'.

Meanwhile, Pakistan has been accused of interfering with investigators' inquiries. As journalist Steve Coll says in a Q&A on The New Yorker's website, 'It's presumed that one reason is that Khan knows quite a lot about how Pakistani generals and other leaders have endorsed or profited from his global trade."

Khan, touted as the father of the Islamic bomb soon after Pakistan went nuclear, is seen as a national hero in Pakistan.

He is also one of the world's most notorious criminals, the former head of a network that distributed nuclear technology on the black market to Iran and North Korea as well as Libya.

The network inspired nightmares for nonproliferation and security officials, and former CIA Director George Tenet even described Khan as 'at least as dangerous as Osama bin Laden', Kaplan said.

"By 2003, Western intelligence officials were onto Khan, and that October they managed to intercept a shipment of centrifuge parts destined for Libya. The seizure marked the beginning of the end for the Khan network," he said.