International Atomic Energy Agency (IAEA) chief Rafael Grossi urged North Korea on Monday to abide by U.N. Security Council resolutions and implement safeguards consistent with the Non-Proliferation Treaty on preventing the spread of nuclear weapons.

Grossi’s statement comes on the heels of the IAEA’s report released late last month that says North Korea restarted the plutonium-producing reactor at its primary Yongbyon nuclear complex to the highest levels in three years.

“The DPRK’s nuclear activities continue to be a cause for serious concern,” he said in a statement. “Furthermore, the new indications of the operation of the 5MW(e) reactor and