North Korea seems to have resumed operations at a nuclear reactor at the controversial Yongbyon nuclear site, the International Atomic Energy Agency (IAEA) says.
The small reactor has a capacity of just 5 megawatts but is able to supply plutonium to make nuclear bombs.
"There were no indications of reactor operation from early December 2018 to the beginning of July 2021," according to an IAEA report on developments in North Korea since September 2020.
"However, since early July 2021, there have been indications, including the discharge of cooling water, consistent with the operation of the reactor," the report said.
South Korea did not initially confirm the IAEA's comments on the reactor.
Seoul is monitoring North Korea's nuclear and missile activities in close cooperation with the U.S., a Unification Ministry spokesperson said on Monday.
The IAEA report was submitted to the agency's board of governors on Friday.
The report also noted that a plant serving a radiochemical laboratory at Yongbyon had been in operation from mid-February to early July. The plant serves as a fuel rod reprocessing facility.
It called the evidence on the processes in the reactor and the laboratory "deeply troubling."
The IAEA bases its observations on purchased images taken by commercial satellites.
Independent verification of nuclear activities in North Korea has not been possible since it expelled international nuclear inspectors from the country a few years ago.
In March, scientists from the Stimson Center in the U.S. who focus on North Korea reported on processes in the laboratory, saying images in January indicated that North Korea might be making preparations to recover plutonium for nuclear weapons.
The reprocessing produces, among other things, plutonium.
North Korea is subject to international sanctions because of its nuclear weapons program, which has been a regular source of concern internationally and, in particular, for regional neighbors like South Korea and Japan.
In February, an internal U.N. report said that North Korea was continuing to push ahead with the program despite the long-standing sanctions.