WASHINGTON, Jan 14: Over the course of three presidential administrations, US governmental officials repeatedly pressed the Chinese government to explain whether it was providing any assistance to Pakistan in the nuclear weapons field , but Chinese officials responded with denials and equivocation, says a new set of secret documents posted on the official website of the US National Archives.

The documents say that recent evidence from Libya of Chinese-language material among the nuclear weapons-design supplied by the Khan network raises new questions about Chinese contribution to Pakistan's nuclear proliferation activities.

Exactly what the US government knew and when it knew it remains highly secret in closed intelligence files, but the newly-available diplomatic record show China refused to cooperate with the US efforts to stifle Pakistan's nuclear programme.

According to these documents, the US government had started expressing unease over China-Pakistan security and military cooperation in the late 1960s. In 1977, US diplomats raised with China the issue of alleged Chinese assistance to Pakistani nuclear-weapons related projects.

A 1982 diplomatic memo shows that China refused to give an 'unequivocal answer' to US queries about nuclear weapons aid to Pakistan. In 1983, the US State Department analysts concluded that China was assisting with the production of fissile materials and possibly with the design of weapons.

In 1989, the George H. W. Bush administration conveyed its concern to China over "reports of Chinese assistance to Pakistan's nuclear weapons programme". A 1989 memo contains denials by Chinese diplomats that China was assisting Pakistan's nuclear programme.

The documents show that in 1992, Washington pressed China to impose full-scope safeguards on the sale of a nuclear reactor to Pakistan because of proliferation concerns.

A late 1992 memo shows more US disquiet over China's "continuing activities with Pakistan's nuclear weapons programs". In 1997, the Clinton administration issued a certification of improvements in Beijing's nuclear proliferation policies.

The posting recalls that recently the Chinese foreign ministry has announced an investigation into the latest charges as well as strengthened commitment to the non-proliferation system.