Sung Kim, director of the State Department's Korea office who led a delegation to the North Korean capital Pyongyang on Thursday, returned to the South by land through a joint security area known as Panmunjom, an AFP photographer on the scene said.

He and three others were carrying a total of seven cardboard boxes which contained some 18,000 pages of documents related to North Korea's plutonium programme.

"We have to take them back and see," Sung Kim told journalists when asked about their contents, seconds after crossing the military demarcation line dividing the two Koreas.

The papers handed by Pyongyang to Sung Kim will be used to help verify an eventual declaration from North Korea on its past nuclear activities, State Department spokesman Sean McCormack has said.

The documentation dating back to 1986 consists of operating records for a five-megawatt reactor and fuel reprocessing plant at the Yongbyon complex where the North had produced its stock of weapons-grade plutonium, the State Department said.

"Review of the operating records... will be an important first step in the process of verifying that North Korea's declaration is complete and correct," the State Department said in a "fact sheet".

"These documents will be examined thoroughly by a team of US verification and other experts," it said.

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" US concerns about the two issues in a confidential separate document to Washington.

The North, which staged its first 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 denuclearisation, the North would receive energy aid, a lifting of US sanctions, the establishment of diplomatic relations with Washington and a formal peace treaty.

Spokesman Moon Tae-Young of the South Korean foreign ministry said Friday North Korea will also present copies of the same documents to China, which in turn will share the information with other participants in six-party talks.

"I think the next round of six party talks will resume late this month or early June," he said.

A senior South Korean government official said the North's delivery of the documents strengthened belief in Pyongyang's sincerity toward the six-party process.

"It is significant that the North yielded information concerning its past nuclear activities that it had adamantly refused to give up," he said.

Sung Kim is expected to meet with South Korea's chief delegate to the six party talks, Kim Sook, on Saturday before returning home on Monday, a South Korean foreign ministry official said.