Sung Kim, director of the State Department's Korea office, returned to the State Department and briefed colleagues there about the documents handed over last week in Pyongyang, the officials said.

"They're going to get started working on these documents, seeing what they have, and leafing through and analyzing 18,000 pages worth of documents, which include records," State Department spokesman Sean McCormack said.

Kim and three colleagues crossed from North Korea into South Korea on Saturday carrying a total of seven cardboard boxes which contained some 18,000 pages of documents related to North Korea's plutonium program.

The papers will be used to help verify an eventual declaration from North Korea on its past nuclear activities, according to the State Department.

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, it 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 denuclearization, the North would receive energy aid, a lifting of US sanctions, the establishment of diplomatic relations with Washington and a formal peace treaty.