The BBC's Jennifer Jensen: "The US Government is on the defensive"

Notra Trulock, head of intelligence at the Department of Energy, said he was told at one point not to pass the information on to Congress, because it could be used to attack US President Bill Clinton's policy on China.

His remarks suggest the White House was told about Chinese espionage much earlier than it has acknowledged.

First public statement

Mr Trulock was one of the main witnesses for Congressman Chris Cox's report on Chinese espionage, which will be made public on Tuesday.

Notra Trulock speaking on NBC's Meet The Press

The allegations that China spied on America's nuclear facilities are well known, but this is the first time Mr Trulock, who has been an important witness to Congress, has spoken in public.

He said he was convinced there had been spying by the Chinese, which he put on a scale with attempts to steal the secret of the atomic bomb during World War II.

"I conclude in my personal view that the potential damage is very significant," he said.

Rob Watson in Washington: "This is the first time Mr Trulock has spoken in public."

He also said he had passed on his concerns to president Clinton's national security adviser in 1996.

Clinton denies knowledge

President Clinton said earlier this year that no-one had reported to him that there may have been espionage at America's laboratories during his time in office.

Bill Richardson: "We don't know whether it came from our labs."

Energy Secretary, Bill Richardson, said he did not believe the president was lying.

"What he was referring to - and this is what he said - was nothing at the nuclear weapons labs.

"There were compromises in the '90s - thefts of nuclear secrets - but we don't know whether it came from our labs."