The five megawatt reactor is capable of producing weapons-grade plutonium, a critical ingredient in building nuclear warheads.
"Since early July 2021, there have been indications, including the discharge of cooling water, consistent with the operation," the IAEA report stated.
Although UN inspectors have been barred from the site since 2009, the IAEA continues to monitor Yongbyon remotely by satellite imagery and other open-source information.
The agency also suggested the attached radiochemical laboratory at Yongbyon had been working since February, likely to reprocess and extract plutonium from spent fuel rods from nuclear power plants.
"[North Korea’s] nuclear activities continue to be a cause for serious concern. Furthermore, the new indications of the operation of the 5-megawatt reactor and the radiochemical laboratory are deeply troubling," the IAEA said.
Yongbyon has been a core part of North Korea’s nuclear programme for decades. Kim Jong-Un, the reclusive dictatorship’s leader, offered to close the facility during negotiations with Donald Trump in 2019, in return for the lifting of international sanctions.
But the then-president rejected the offer, reportedly because it would only represent ending part of North Korea’s nuclear capability.
There are believed to be several other uranium processing facilities – producing the other critical fuel needed for nuclear weapons – across the country.
The IAEA report also notes signs of mining and concentration activities at a uranium mine and plant in Pyongsan, and a suspected enrichment facility in Kangson.
"There has been no agreement governing these facilities for a long time now," Joshua Pollack, a researcher at the James Martin Center for Nonproliferation Studies, explained.
“North Korea’s appetite for warheads is not yet sated, it seems."
Leading figures from the regime, including Mr Kim and his increasingly powerful sister Kim Yo-Jong, have made belligerent threats against the United States in recent months and pledged to strengthen the country’s “absolute deterrence” and speed up weapons development.
Joe Biden’s new administration has said it wants to reopen talks with North Korea about denuclearisation but Pyongyang has so far rebuffed the offers.
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