A U.S. nuclear envoy returned from North Korea on Saturday, shuttling boxes of documents detailing Pyongyang's production of arms-grade plutonium across the Cold War border dividing the two Koreas, Reuters reported.
U.S. officials have said turning over the 18,000 to 19,000 pages of documents would bring North Korea closer to the goal set in an international disarmament deal of declaring all its nuclear activities.
The North missed an end of 2007 deadline to provide the atomic inventory.
U.S. State Department envoy Sung Kim, who left for Pyongyang on Thursday, arrived in a motorcade from the North and then joined a separate motorcade waiting for him in the South, an official with the U.S. embassy in Seoul said.
Kim spoke briefly and said officials now needed to investigate the content of the documents that were carried across the border in boxes.
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.
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.
A senior U.S. official, who asked not to be named, has said the documents provided detailed logs of how much plutonium was produced by North Korea. Officials in Seoul and Washington have said the next step is verification.
North Korea, which tested a nuclear device in October 2006, is thought by Washington to have produced about 50 kg (110 lbs) of plutonium, which proliferation experts said is about enough for about eight nuclear bombs.
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.
U.S. President George W. Bush said in late April 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 has yet to respond to White House charges that North Korea was helping Syria build the reactor that could produce arms-grade plutonium.