The Washington Post, citing government officials and arms experts on Sunday, said the documents, some of which included text in Chinese, contained detailed, step-by-step instructions for assembling an implosion-type nuclear bomb that could fit atop a large ballistic missile.

They also included technical instructions for manufacturing components for the device.

The designs were sold to Libya by a Pakistani-led trading network through a nuclear trading network that is now the focus of an expanding international probe, the daily reported.

"It was just what you'd have on the factory floor. It tells you what torque to use on the bolts and what glue to use on the parts," one weapons expert who had reviewed the blueprints told The Post. He described the designs as "very, very old" but "very well engineered."

Dismantling

US intelligence officials concluded years ago that China aided Pakistan in building its first nuclear weapon until the 1980s.

China's actions "were irresponsible and short-sighted, and raise questions about what else China provided to Pakistan's nuclear programme," David Albright, a nuclear physicist and former UN weapons inspector in Iraq, told The Post.

Libya announced on 19 December it had agreed to disclose and dismantle its biological, chemical and nuclear weapons programmes, a surprise move that came after months of secret negotiations between Tripoli, London and Washington.

Pakistan's top nuclear scientist later admitted selling nuclear secrets to Libya, North Korea and Iran.