North Korea appears to have restarted its primary nuclear reactor for the first time in three years, according to a new report from the International Atomic Energy Agency (IAEA), with the nuclear watchdog calling the development “deeply troubling.”

The reported activity since early July comes after North Korean leader Kim Jong Un said in January that the country would “push ahead” with developing “super-sized nuclear warheads,” “tactical nuclear weapons” and an “underwater-launch nuclear strategic weapon.”