A published report adds further complications to the North Korean nuclear picture, alleging that Pyongyang may have a second, secret facility for producing weapons-grade plutonium. This report comes days before the 50th anniversary of the truce agreement that ended the Korean War.

The New York Times newspaper Sunday reports U.S. officials confirm that sensors on North Korea's borders have begun to detect elevated levels of krypton-85, a gas emitted as spent fuel is converted into plutonium.

The report says the issue that most concerns American and Asian officials, though, is analysis showing that the gas is not coming from North Korea's main nuclear plant, Yongbyon. Instead, the experts believe the gas may be coming from another hidden facility, buried deep in the mountains.

The Times points to one American intelligence estimate as saying North Korea has as many as 15,000 underground military-industrial sites.

A senior U.S. administration official is quoted as saying the new evidence is "very worrisome, but still not conclusive." The news comes less than two weeks after North Korea claimed it had completed reprocessing 8,000 spent nuclear fuel rods into plutonium. This would be enough material to make several nuclear bombs.

The news also comes as both countries prepare to mark the 50th anniversary of the signing of the armistice that brought the three-year Korean War to an end.

Official North Korean media Sunday pointed to recently announced U.S. plans to re-distribute American troops in South Korea as a provocation, and accused the United States of planning another Korean War. According to the North Korean reports, American forces are "watching for an opportunity with their fingers locked on the trigger."

The latest nuclear dispute between Washington and Pyongyang flared in October, when U.S. officials said North Korea admitted having a clandestine, uranium-based nuclear program in violation of international agreements.