The disclosure is part of a six-nation agreement reached in October in which Pyongyang said it would shut down its nuclear power plant by the end of the year in exchange for energy and other aid.

In Seoul, South Korean Foreign Minister Song Min-soon called Pyongyang's silence on its atomic history a "bump" in the talks between the two Koreas, China, Japan, Russia and the United States, the Yonhap news agency said.

Last week, Christopher Hill, the chief U.S. nuclear envoy, traveled to Pyongyang to emphasize part of the agreement was disclosure, as he said the United States had "credible evidence" of a uranium enrichment program in North Korea that was capable of manufacturing warheads.

North Korea has repeatedly denied U.S. claims that surfaced in 2002 that it was manufacturing nuclear weapons.