North Korea has handed over more than 18,000 pages of records on its nuclear weapons programme to the US.
The documents log activities at North Korea's plutonium reactor at Yongbyon, which is thought to have produced the material for nuclear tests in 2006.
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A US negotiator was due to take the documents to Washington to be examined.
North Korea agreed in February 2007 to give up its nuclear weapons programme in return for large-scale aid and the lifting of sanctions.
The deal was agreed following six-party talks between China, the US, Japan, Russia and North and South Korea.
Sceptics
The BBC's John Sudworth reports from South Korea that the Yongbyon documents will provide an important glimpse into North Korea's bomb-making past, though some will question just how much new information they will contain.
Doubts over North Korea's other nuclear activities remain, in particular its alleged secret uranium enrichment programme and the transfer of nuclear technology to Syria, he says.
Those are the issues that have been holding up progress on last year's agreement.
North Korea has reportedly agreed to a compromise solution under which it will merely acknowledge US concerns.
Some Republicans in Washington were sceptical about the importance of the documents.
"Any mediocre performance by North Korea is taken as an earth-shattering positive development by our state department," said Congressman Peter Hoekstra.
US state department spokesman Sean McCormack said that once the documents were back in Washington "our top three priorities are going to be verification, verification, verification".
(BBC)
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