Until the mid-1990s China lagged several years behind the US in nuclear weapons technology. It did not, for example, have the ability to manufacture small warheads which could be launched at multiple targets from one missile. But in May 1995 China conducted an underground nuclear test in the north-western province of Xinjiang, and another in June 1996. According to US intelligence reports the test results had similarities to the most advanced US miniature warhead, the W-88.
China has reacted angrily to charges made by US officials, since the story broke in the New York Times on Sunday, that it stole the technology to close the missile generation gap. The charges were "baseless" and "irresponsible" rumours spread by people who want to stop bilateral ties from improving, China's Foreign Ministry spokesman, Mr Zhu Bangzao, told a news briefing in Beijing yesterday.
Mr Zhu had been asked to comment on reports that the Los Alamos National Laboratory in the US had dismissed a Chinese-American scientist named Wen Ho Lee, suspected of leaking the nuclear secrets to Beijing.
"Some people want to obstruct the United States from exporting normal high technology to China or some people want to impede Sino-US ties from improving and developing," Mr Zhu said, adding however, "We are paying attention to these reports. We are still looking into the situation."
The issue has worsened strains between Washington and Beijing over human rights, trade and Taiwan, just 10 months after a visit to China by President Clinton, though both sides are still committed to improving ties. The Foreign Ministry spokesman expressed optimism that the visit to Washington next month by the Chinese Prime Minister, Mr Zhu Rongji, "will further improve understanding and expand wide-ranging co-operation between the two sides and push forward development of healthy and stable bilateral relations".
Last week the US Secretary of State, Ms Madeleine Albright, also stressed the advantages of collaboration over confrontation.
One of the reasons the US wishes to avoid a breakdown in relations with China is the prospect that Beijing would resume supplying weapons technology to third countries hostile to the US, observers said.
The CIA reported in 1996 that China was the most significant world supplier of weapons of mass destruction, related goods and technology to foreign countries and "provided a tremendous variety of assistance to both Iran's and Pakistan's ballistic missile programmes". It said China also was the primary source of nuclear-related equipment and technology to Pakistan and a key supplier to Iran. Since then Washington officials have said that this is no longer the case.
Plans by the US to place parts of Asia, including Taiwan, under a missile shield are also aggravating relations between the two powers. China was "resolutely opposed" to including Taiwan under the Theatre Missile Defence system (TMD) because it would delay reunification, Mr Zhu Bangzao said yesterday.
Beijing considers Taiwan a breakaway province, and has made reunification its main policy goal. The TMD system, which evolved from the aborted "Star Wars" anti-missile programme of the 1980s, is being discussed by the US, Japan and South Korea. Washington says it is considering including Taiwan under the umbrella because of a perceived missile threat from North Korea. But the system is some way from being installed. And Ms Albright said that China should stop worrying "about a decision that has not been made to deploy defensive technologies that do not yet exist."
However, on Sunday China's Foreign Minister, Mr Tang Jiaxun, hardened Beijing's opposition to the idea and warned of the consequences of the US going ahead. He told reporters: "If some people intend to include Taiwan under Theatre Missile Defence, that would amount to an encroachment on China's sovereignty and territorial integrity and also be an obstruction to the great cause of peaceful reunification to the motherland."
He added that the development and research of TMD would "exert a negative impact on the global and regional strategic balance and stability into the next century".
A recently published Pentagon report said China was building a major force of ballistic and cruise missiles near its coasts and could present an overwhelming military threat to Taiwan in five years. The Foreign Ministry spokesman denied yesterday that there was any build-up of missiles in the zone.