The United States has said documents it has received from North Korea are an "important first step" towards full disclosure of the country's nuclear activities.
A US envoy arrived in South Korea on Saturday with seven cardboard boxes containing 18,000 pages of information on a weapons-grade plutonium programme dating back to 1986.
The US state department said in a statement that the documents will be used to verify Pyongyang's public comments on past nuclear activities, and that it would "continue to press the DPRK to fulfill its declaration commitment".
North Korea agreed to dismantle and fully disclose it nuclear activities, in exchange for economic and political rewards, by the end of last year.
Sung Kim, the US state department envoy who was given the sensitive records, will return on Monday to Washington, where the documents are to be "examined thoroughly" by a nuclear verification team and other experts.
Full disclosure
Under a six-country deal, North Korea was required to provide a full accounting of its fissile material and nuclear weaponry, as well as answer US suspicions that it enriched uranium for weapons and proliferated technology to Syria.
In exchange for the declaration, the US has promised to remove North Korea from its "terrorism blacklist" and remove sanctions that restrict Pyongyang from having access to international finance.
Since November, US experts have been on the ground at Yongbyon overseeing disablement of the reactor.
The US state department says eight-out-of-11 agreed disablement activities at three core facilities on the complex have been completed and that more than one-third of spent fuel rods have now been discharged.
The main sticking point in the declaration has been Pyongyang's reluctance to discuss any transfer of nuclear technology to other countries, notably Syria, as well as its suspected pursuit of uranium enrichment.
George Bush, the US president, said in late April that he released intelligence about the suspected North Korea-Syria nuclear collusion to put pressure on Pyongyang to come clean on all its nuclear activities.
Pyongyang denies both charges of uranium enrichment and technology proliferation.
Under a reported deal, it will merely "acknowledge" US concerns about the two issues in a confidential separate document to Washington.