Diplomats say that North Korea has made all of its Yongbyon nuclear facilities off limits to international inspectors.
The diplomats say the North's decision was made recently but declined to offer details. The diplomats demanded anonymity Thursday because their information was confidential.
The reported move expands the area that the International Atomic Energy Agency inspectors are no longer allowed to monitor.
Pyongyang already barred agency personnel from its plutonium reprocessing facility at Yongbyon last month, when it made good on threats to restart its weapons-producing atomic program.
Japan's Kyodo news agency also reported Thursday that the United States has told Japan it will remove North Korea from its terrorist blacklist this month.