In its first direct response to the Cox report, the Chinese Foreign Ministry Spokesman, Zhu Bangzao, described the allegations as absurd and without basis.

Mr Zhu accused the authors of the report of staging an anti-China farce and trying to damage bilateral relations. The statement asserted that China had always protected its national security through its own efforts.

Defensive purposes

The Chinese embassy in Washington stressed that China's nuclear capability was purely for defensive purposes. Officials accused the US of underestimating the abilities of China's scientists to do their own research.

But they have also made it clear that Beijing still desires good relations with Washington, and the attack on the Cox report was not given headline status in state television or press reports.

China's official news agency has also highlighted President Clinton's insistence that he will not change his policy of engagement with Beijing.

However, the BBC's Beijing Correspondent, Duncan Hewitt, says that in the current climate of Sino-US relations, the report is clearly fuelling suspicions in China, that appear almost as strong as those in Washington.

The foreign ministry statement again accused the authors of the Cox report of trying to distract attention from Nato's bombing of the Chinese embassy in Belgrade.

Clinton pledge

President Clinton has meanwhile promised to do more to protect American nuclear secrets, while defending his policy of engagement with Beijing as being in the national interest.

President Clinton says he was never told of the leaks

"We have a solemn obligation to protect such national security information and we have to do more to do it," the president told an audience in Texas.

"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.

Tom Carver in Washington: China has 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 thermo-nuclear 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.

But the government has challenged some of the 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.

The bipartisan report said the stolen information had allowed China to jump from 1950s to 1990s technology in a matter of years.

Stolen information included details on:

• US thermonuclear warheads, including the W-88 - the most sophisticated weapon the US has ever built.

• An enhanced neutron bomb that has never been deployed by any country.

• Developmental technology that could be used to attack US satellites and submarines.

The report adds that one submarine-based and two land-based weapon systems have the capability to reach the US mainland.

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.

Beijing primarily focused on Los Alamos, Lawrence Livermore, Oak Ridge and Sandia national laboratories.

China obtained the information using direct spying and "front companies" in the US, the report says.

The report also picks out two US companies - Loral and Hughes Aircraft - which went "outside the scope" of their export licences to provide China with information, helping to improve the reliability of nuclear missiles.