The investigation follows reports that China obtained information on building multiple-warhead nuclear weapons from a Taiwanese-born scientist who worked at the US National Laboratories in Los Alamos, New Mexico.

Mr Clinton: Keen to forge ties with Beijing

China has yet to field such a weapon, but a prototype bears a strong resemblance to the W-88, a US warhead.

US intelligence officials believe it could be the most serious spying case in a generation, saving China about 15 years of its own research.

Retired Navy Admiral David Jeremiah will lead the independent inquiry which will act as a watchdog for the official intelligence investigation set to end this month.

Tall tales

Republicans have accused the Clinton administration of failing to react quickly to prevent further damage after learning of the apparent leak in 1996.

They say the president was slow to act because he did not want to upset ties with China.

Chinese Premier Zhu Rongji, due to visit Washington next month, has called the allegations a "tale from the Arabian Nights".

But the chairman of the US Senate Intelligence Committee on Monday proposed a moratorium on scientific visits between US national research laboratories and "sensitive" countries.

"I believe myself that this is perhaps just the tip of an iceberg," Republican Senator Richard Shelby said after a meeting with CIA Director George Tenet about the Los Alamos case.

"I believe that the President of the United States and perhaps the secretary of energy should put a moratorium on the exchange of people coming into our labs and our scientists going to their labs and perhaps giving them information.

"As we stand here, our labs basically are not as secure as they should be," he added. "Our national security is too important to ignore that.''

Lie detector test

The Taiwanese-born computer scientist at the centre of the allegations, Wen Ho Lee, was fired from his job at Los Alamos a week ago.

He recently failed a lie detector test as part of a three-year investigation into the alleged disclosure of sensitive nuclear missile technology to China in the mid-1980s.

Mr Lee has not been charged with any crime, and Newsweek magazine said the FBI now believes it has virtually no chance of making a case against him.