WASHINGTON (AFP) - A US envoy was due to leave North Korea Friday with documents about its plutonium program amid efforts to give it a clean bill of health on nuclear disarmament, a top US official said.

The papers being handed over to nuclear expert Sung Kim will be used in the process to verify an eventual declaration from North Korea on its past nuclear activities, State Department spokesman Sean McCormack told reporters.

"He's going to bring with him a significant number of documents related to North Korea's plutonium program. And we'll have an opportunity in the coming days and weeks to assess the significance of these documents," he said.

Kim, director of the State Department's Korea office who led a delegation to the North Korean capital Pyongyang on Thursday, was due to leave Friday and return to the United States, McCormack said.

Both sides reported progress after Kim visited the North last month.

The North, which staged a nuclear test in October 2006, is disabling its plutonium-producing reactor and other plants under a deal reached last year with the United States, China, Japan, Russia and South Korea.

But disputes over the declaration due last December 31 have blocked the start of the final phase of the process -- the permanent dismantling of the plants and the handover of all material.

The declaration is crucial to verifying that all material, including stockpiled plutonium which could be used for bomb-making, is accounted for.

In return for total denuclearization, the North would receive energy aid, a lifting of US 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 an alleged secret uranium enrichment program and about suspected involvement in building the plant in Syria.

The North denies both activities. Under a reported deal, it will merely "acknowledge" US concerns about the two issues in a confidential separate document to Washington.

The main declaration, to be given to talks host China, would detail the plutonium operation.

McCormack said Kim will "be consulting very closely" about the documents with his partners in the six-party process and "sharing the information."

The State Department spokesman said meanwhile that a US government team had left Pyongyang following what he said were "inconclusive" discussions about delivering food aid to the impoverished Stalinist nation.

"They went there to take a look at whether or not conditions had changed sufficiently so that we could in good conscience and good faith provide food aid and know that was going to get to people who need it," McCormack said.

The State Department has said the provision of food aid depends on the level of need, supply "and our view of other needs that might exist, and our ability to ensure aid is reliably reaching the people in need."

North Korea said the talks with the team had gone well "in a sincere atmosphere."

A leading US research institute warned last week that the North again runs the risk of outright famine, 10 years after up to one million of its people died of starvation.

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