Documents turned over by Libya contain evidence of China's role in transferring nuclear technology to Pakistan in the early 1980s, the Post said, citing government officials and arms experts.
The Chinese designs were sold to Libya by a Pakistani-led trading network that is the target of an international investigation, the Post said. The report did not say whether Chinese officials were believed to have known of the transfer to Libya.
China's actions "were irresponsible and short-sighted, and raise questions about what else China provided to Pakistan's nuclear program," the newspaper quoted David Albright, a nuclear physicist and former U.N. weapons inspector in Iraq, as saying. "These documents also raise questions about whether Iran, North Korea and perhaps others received these documents from Pakistanis or their agents."
The Chinese Foreign Ministry did not respond Sunday to a request for comment on the report. China is Pakistan's main arms supplier and one of its closest military allies.
The documents were turned over to U.S. officials in November following Libyan leader Moammar Gaddafi's decision to renounce weapons of mass destruction and open his country's weapons laboratories to international inspection, the Post said.
It said they had been analyzed by experts from the United States, Britain and the Vienna-based International Atomic Energy Agency, the U.N. nuclear watchdog.
The founder of Pakistan's nuclear weapons program, Abdul Qadeer Khan, admitted in a televised confession last month that he passed nuclear secrets to North Korea, Libya and Iran.
Pakistan exploded its first nuclear bomb in 1998, but it was a more modern design than the one sold to Libya, Albright told the Post.
The Libyan documents, some of which included text in Chinese, contained step-by-step instructions for assembling an implosion-type, 1,000-pound nuclear bomb that could be carried by a large ballistic missile as well as technical instructions for manufacturing components for the device, the Post said, citing unidentified officials and experts.
The newspaper noted that Libya did not possess a missile large enough for the weapon.
But Albright told the Post that the bomb blueprints in the Libyan documents "would be highly useful to countries such as Iran and North Korea" which had more developed ballistic missile programs.
The newspaper quoted officials familiar with the documents as saying the device depicted in the blueprints appears similar to a weapon tested by China in the 1960s. One Europe-based weapons expert told the newspaper that such a relatively simple design might be coveted by terrorist groups and could be carried in a pickup truck.
Jonathan Wolfsthal, a nonproliferation expert at the Carnegie Endowment for International Peace, told the Post that China's views on proliferation have changed dramatically since the 1980s, and its leaders are now more cooperative in working with other countries to stop the spread of nuclear weapons technology.