PANMUNJOM, South Korea, May 10 (Reuters) - A U.S. nuclear envoy returned from North Korea on Saturday, shuttling boxes of documents across the Cold War border dividing the two Koreas that detail Pyongyang's production of arms-grade plutonium.

U.S. officials have said turning over the some 18,000 to 19,000 pages of documents would bring North Korea closer to its goal of declaring its nuclear activities. The North missed an end of 2007 deadline set in an international disarmament deal 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. (Reporting by Jon Herskovitz; Editing by Alex Richardson)

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