U.S. officials say they plan to thoroughly examine 18-thousand documents on North Korea's nuclear program, which Pyongyang has turned over to a U.S. envoy.

The U.S. State Department's director of Korean affairs, Sung Kim, traveled from North to South Korea Saturday with boxes of documents detailing two decades of North Korea's plutonium enrichment efforts.

He is to return to Washington Monday with the information.

The U.S. official led a five-member negotiating team that arrived in Pyongyang May 8 with the intent of convincing North Korea to fully declare its nuclear activities. In exchange, the United States has pledged food and other aid to the impoverished country.

The U.S. envoy's visit was his second in two weeks. Kim said he hoped to help resolve the dispute over Pyongyang's failure to meet a December 31, 2007 deadline to disclose all of its nuclear activities.

North Korea agreed to that deadline as part of six-nation nuclear disarmament deal. Under the agreement, China is to review Pyongyang's official declaration of its nuclear activities.

North and South Korea, the United States, China, Japan and Russia are members of the six-party nuclear talks.