An American diplomat left North Korea today with boxes of documents containing details of two decades of Pyongyang's nuclear secrets.

Sung Kim, the US State Department’s senior North Korean specialist, returned to South Korea after collecting approximately 18,000 secret papers during a three-day visit to the Yongbyon nuclear reactor that has been at the heart of the secretive communist country’s nuclear weapons program.

Washington has accused North Korea, which conducted its first nuclear test in 2006, of refusing to come clean over its uranium-based nuclear programme and suspicions that it had transferred nuclear technology to Syria.

A State Department statement said North Korea, working under continuous US oversight since November, has completed eight of 11 promised steps to disable and mothball the reactor, with American experts overseeing the discharge of spent fuel rods - a project that is about one-third complete.

“These actions have halted (North Korea’s) ability to produce additional weapons-grade plutonium for its nuclear weapons program,” the statement said.

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