A U.S. diplomat left North Korea on Saturday with boxes of documents detailing activities at the nuclear reactor that is at the heart of the communist country's nuclear weapons program.
Washington plans to scrutinize the technical logs from the Yongbyon reactor to see if the North is telling the truth about a bomb program that it has agreed to trade away for economic and political rewards.
Sung Kim, the U.S. State Department's top Korea specialist, returned to South Korea by land across the heavily fortified border after collecting approximately 18,000 secret papers during a three-day visit to Pyongyang.
Kim and four accompanying officials crossed the border at the truce village of Panmunjom, inside the Demilitarized Zone separating the two Koreas. They carried seven cardboard boxes apparently containing the documents.
"We have to take them back and see," Kim told reporters without elaborating, according to South Korea's Yonhap news agency.
Kim will arrive Washington on Monday, said U.S. Embassy spokesman Max Kwak.
The North's handover of the sensitive records came as last year's disarmament-for-aid deal remained stalled due to Pyongyang's failure to fully disclose its nuclear programs. Washington has accused the North, which conducted its first nuclear test in 2006, of refusing to address suspicions that it pursued a uranium-based nuclear program and transferred nuclear technology to Syria.
Washington and Pyongyang agreed last month to break the impasse in a way that requires North Korea to acknowledge those concerns and to set up a system to verify that the country does not conduct such activities in the future.
The U.S. scrutiny of the North Korean records was expected to focus on the amount of plutonium — a key nuclear bomb ingredient — that the North has produced from spent fuel from the Yongbyon reactor. The reactor has been shuttered and was being disabled under last year's agreement.
If the records are verified and Pyongyang submits a nuclear declaration, the U.S. was expected to start taking North Korea off its terrorism and economic sanctions blacklists — a concession promised under last year's deal.
In Washington the State Department said the records date back to 1986 and were expected to cover reactor operations and nuclear reprocessing activity.
A statement from the department said North Korea, working under U.S. oversight since November, has completed eight of 11 promised steps to mothball the reactor.
"These actions have halted (North Korea's) ability to produce additional weapons-grade plutonium for its nuclear weapons program," the statement said.