The UN nuclear watchdog agency told its 35-nation board on Thursday that it has reason to believe North Korea is restarting a nuclear reactor, according to remarks from Yukiya Amano, chief the International Atomic Energy Agency.
"Activities have been observed at the site that are consistent with an effort to restart the 5MW(e) reactor," Amano told the IAEA's board.
"However, as the Agency has no access to the site, it is not possible for us to conclusively determine whether the reactor has been re-started," he added.
In April, North Korea said it would restart its Yongbyon nuclear complex, which, according to some experts, may be able to produce nuclear material for atomic bombs. However, Pyongyang indicated the action was for deterrent purposes.
The IAEA has an extremely limited ability to monitor North Korea's nuclear activity, especially since the hermit nation kicked out inspectors in 2009.
"The Agency continues to monitor developments at the Yongbyon site, mainly through satellite imagery," Amano said.
However, South Korea's National Intelligence Service - a spy agency - had said last month that North Korea had restarted nuclear activity, according to the Economic Times.
In addition, the US-Korea Institute at the Johns Hopkins School of Advanced International Studies said in September that satellite imagery indicated North Korea had restarted the facility, according to Reuters.
Some experts believe North Korea has the ability to produce nuclear bombs. But it doesn't appear their military has the technology to weaponize them into long-range rockets.
"I remain seriously concerned" about North Korea's program, Amano said.