"We still await more complete results and a more complete report as to exactly what happened," Negroponte told reporters after briefing Japanese Foreign Minister Masahiko Komura on developments.
U.S. nuclear expert Sung Kim was leaving North Korea on Friday with papers which will be used to help verify an eventual declaration from North Korea on its past nuclear activities, the U.S. State Department said.
"I know he's bringing back some records that have to do with the nuclear production programs of North Korea," Negroponte said.
But he was cautious over the papers after North Korea in December missed a key deadline in a six-nation deal to make a declaration on all nuclear material, including stockpiled plutonium which could be used for bomb making. "This is part of the ongoing process connected with the six-party talks and leading hopefully to the full denuclearization of the Korean peninsula," he said.
"So I would say this was a step in a process but it's an ongoing process," he added.
The North, which staged a plutonium-based nuclear test in October 2006, is disabling its plutonium-producing reactor and other plants under a deal reached in February 2007 with the United States, China, Japan, Russia and South Korea.
In return for total denuclearization, the North would receive energy aid, a lifting of U.S. sanctions, the establishment of diplomatic relations with Washington and a formal peace treaty.
In addition to the declared plutonium operation, Washington said the declaration must clear up suspicions about alleged secret uranium enrichment and about suspected proliferation to Syria.
The North denies both activities. Under a reported deal, it will merely "acknowledge" U.S. concerns about the two issues in a confidential separate document to Washington.
