But he defended his policy of engagement with Beijing as being in the national interest.

Tom Carver in Washington: China managed to acquire missile blueprints

China is now thought to possess detailed information on every nuclear warhead in the US arsenal and the neutron bomb.

Beijing could begin testing advanced thermonuclear weapons as early as this year and deploy them by the year 2002, says the report.

But despite high-level knowledge of the thefts, security at United States nuclear laboratories still "does not meet even minimal standards", it adds.

The three-year investigation concludes that despite high-level knowledge of the leaks, little was done to tighten controls.

President Clinton responded by saying his administration was already "moving aggressively to tighten security" at US nuclear laboratories.

President Clinton says he was never told of the leaks

Speaking to an audience in Texas, he said:"I want to assure you and all the American people that I will work very hard with the Congress to protect our national security, to implement the recommendations (of the Congressional report) and to continue our policy of engagement because both of them are in the national interest," Mr Clinton said.

Thirty of the committee's 36 recommendations have already been accepted by the White House.

President Clinton says he has already approved

• a 15-fold increase in the counter-intelligence budget
• lie-detector tests for American nuclear scientists and
• background checks on colleagues visiting from overseas.

But he said that without his policy of constructive engagement, Beijing would not have been brought into efforts to stop the spread of nuclear weapons.

China still 'far behind'

Government officials have challenged some of the report's findings. Energy Secretary Bill Richardson denied the assertion that Chinese nuclear technology was now up to US standards.

"China is not up to par with the United States on nuclear development. It is far behind us. There is no evidence that that is changing," he added.

And he said that the report was completed last year and did not include "dramatic steps" taken since last autumn to improve security.

"We have plugged the leaks. What I am saying is we've corrected the problems," he told CNN.

Allegations 'groundless'

The BBC's Duncan Hewitt in Beijing: Tensions look set to grow

Beijing has rejected the report as "groundless" and "highly exaggerated". It accuses the US of faking information in order to damage China's credibility.

Congressman Chris Cox, who led the investigation, said China could also pass on the technology to regimes which were less stable.

Many analysts say the case appears to be the most serious breach of nuclear security since the Soviet Union stole atom bomb secrets in the 1940s.

Cohen drops China trip

Just hours after the report's publication the Pentagon announced that US Defence Secretary William Cohen had dropped plans to visit China next month because of heightened tensions between the two countries.

"It probably is not the easiest time to visit China," his spokesman Kenneth Bacon said, alluding to the spy allegations and the mistaken Nato bombing of the Chinese Embassy in Belgrade.

And in another development likely to sharpen Sino-American tensions, the House of Representatives has voted unanimously for a resolution commemorating the 10th anniversary of China's suppression of pro-democracy demonstrations in Tiananmen Square in Bejing.