The United States said on Saturday documents handed over by North Korea detailed its weapons-grade plutonium programme as far back as 1986 and were an "important first step" in getting a full declaration of the North's nuclear activities.
In a "fact sheet" providing limited details of the documents, the State Department said the 18,000 pages covered three major periods when plutonium was produced by North Korea for nuclear weapons.
"The United States and the other parties continue to press the DPRK to fulfil its declaration commitment," said the statement, referring to Pyongyang's failure to produce a full declaration of its nuclear activities by the end of last year.
The documents were given by Pyongyang to the State Department's Korea expert, Sung Kim, on Thursday who then hand-carried them over the heavily fortified demilitarised zone into South Korea on Saturday.
Kim spoke briefly and said officials now needed to investigate the contents of the documents. The State Department said seven large boxes were carried across the border.
He will return to Washington on Monday where the State Department said the documents would be "examined thoroughly" by a U.S. nuclear verification team and other experts.
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 U.S. suspicions that it enriched uranium for weapons and proliferated technology to Syria.
North Korea is expected to submit a 40 to 50-page report on its nuclear activities in the next few weeks to China, the host of the six-way nuclear disarmament talks, South Korea's Yonhap news agency reported on Saturday, citing diplomatic sources.
If North Korea makes the declaration, the United States has promised to take it off its terrorism blacklist and remove sanctions that restrict Pyongyang from tapping into international finance.
