The terms of a secret deal under which China transferred 50 kg of uranium to Pakistan in 1982 for making two atomic bombs were set in a mid-1976 conversation between Mao Zedong and then Pakistan prime minister Zulfikar Ali Bhutto, a leading US daily reported on Friday.
Citing accounts of controversial Pakistani nuclear scientist A.Q. Khan, The Washington Post said Khan - then a metallurgist working at a Dutch centrifuge manufacturer - was provoked to offer his services to Bhutto by India testing its first nuclear bomb two years earlier.
Khan, according to the US daily, said he and two other Pakistani officials - including then foreign secretary Agha Shahi, since deceased - worked out the details when they travelled to Beijing later that year for Mao's funeral.
Over several days, Khan was cited as saying, he briefed three top Chinese nuclear weapons officials - Liu Wei, Li Jue and Jiang Shengjie - on how the European-designed centrifuges could swiftly aid China's lagging uranium-enrichment programme.
China's foreign ministry did not respond to questions about the officials' roles, the Post said.
"Chinese experts started coming regularly to learn the whole technology" from Pakistan, Khan states, staying in a guesthouse built for them at his centrifuge research centre.
Pakistani experts were dispatched to Hanzhong in central China, where they helped "put up a centrifuge plant", Khan said in an account he gave to his wife after coming under government pressure, according to the Post.
"We sent 135 C-130 planeloads of machines, inverters, valves, flow meters, pressure gauges," he wrote. "Our teams stayed there for weeks to help and their teams stayed here for weeks at a time."
In return, China sent Pakistan 15 tonnes of uranium hexafluoride (UF6), a feedstock for Pakistan's centrifuges that Khan's colleagues were having difficulty producing on their own.
Khan was quoted as saying the gas enabled the laboratory to begin producing bomb-grade uranium in 1982. Chinese scientists helped the Pakistanis solve other nuclear weapons challenges, but as their competence rose, so did the fear of top Pakistani officials that Israel or India might pre-emptively strike key nuclear sites.
Mohammed Zia ul-Haq, Pakistan's former military ruler, "was worried", Khan said, and so he and a Pakistani general who helped oversee the nation's nuclear laboratories, were dispatched to Beijing with a request in mid-1982 to borrow enough bomb-grade uranium for a few weapons.
After winning Chinese leader Deng Xiaoping's approval, Khan, the general and two others flew aboard a Pakistani C-130 to Urumqi.
Khan says they enjoyed barbecued lamb while waiting for the Chinese military to pack the small uranium bricks into lead-lined boxes, 10 single-kilogram ingots to a box, for the flight to Islamabad.
According to Khan's account, cited by the Post, Pakistan's nuclear scientists, however, kept the Chinese material in storage until 1985, by which time the Pakistanis had made a few bombs with their own uranium.
Khan said he got Zia's approval to ask the Chinese whether they wanted their high-enriched uranium back. After a few days, they responded "that the HEU loaned earlier was now to be considered as a gift ... in gratitude" for Pakistani help, Khan said.
The laboratory promptly fabricated hemispheres for two weapons and added them to Pakistan's arsenal, he was quoted as saying.