In 1982, a Pakistani military C-130 left the Chinese city of Urumqi with a highly unusual cargo: enough weapons-grade uranium for two atomic bombs, according to accounts written by the father of Pakistan’s nuclear weapons programme, Abdul Qadeer Khan, and provided to The Washington Post.

The uranium transfer in five stainless-steel boxes was part of a broad-ranging, secret nuclear deal approved years earlier by Mao Zedong and Zulfiqar Ali Bhutto that culminated in an exceptional act of proliferation by a nuclear power, according to the accounts by Khan, who is under house arrest.

US officials say they have known about the transfer for decades and once privately confronted the Chinese — who denied it — but have never raised the issue in public or sought to impose direct sanctions on China for it. US President Barack Obama, who said in April that “the world must stand together to prevent the spread of these weapons”, plans to discuss nuclear proliferation issues while visiting Beijing on Tuesday.

According to Khan, the cargo came with a blueprint for a simple weapon that China had already tested, supplying a virtual do-it-yourself kit that significantly speeded Pakistan’s bomb effort. The transfer also started a chain of proliferation: US officials worry that Khan later shared related Chinese design information with Iran; in 2003.

Although Chinese officials have for a quarter-century denied helping any nation attain nuclear capability, current and former US officials say Khan’s accounts confirm that China provided such assistance.

“Upon my personal request, the Chinese Minister... had gifted us 50 kg of weapon-grade enriched uranium, enough for two weapons,” Khan wrote in a 11-page narrative that he prepared after his January 2004 detention for unauthorised nuclear commerce. “The Chinese gave us drawings of the nuclear weapon, gave us 50 kg enriched uranium,” he said in a separate account sent to his wife several months earlier.

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