North Korea appears to have restarted the operation of its main nuclear reactor used to produce weapons fuels, the UN atomic agency said, as the North openly threatens to enlarge its nuclear arsenal amid long-dormant nuclear diplomacy with the United States.

The annual report by the International Atomic Energy Agency refers to a 5-megawatt reactor at the North’s main nuclear complex in Yongbyon, north of Pyongyang. The reactor produces plutonium, one of the two key ingredients used to build nuclear weapons along with highly enriched uranium.

“Since early July 2021, there have been indications, including the discharge of cooling water, consistent with the operation of the reactor,” said the IAEA report dated on Friday.

The report said there were indications of the operation of Yonbyon’s radiochemical laboratory from mid-February to early July this year. It said this period of operation is consistent with previous reprocessing campaigns announced by N Korea of irradiated fuel discharged from the reactor. The laboratory is a facility where plutonium is extracted by reprocessing spent fuel rods removed from reactors.

 “(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.

The IAEA has not had access to Yongbyon or other locations in North Korea since the country kicked out IAEA inspectors in 2009. The agency said it uses satellite imagery and open source information to monitor developments in North Korea’s nuclear program.