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

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.

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

In its article, The Indian Express said that 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 North Korea of irradiated fuel discharged from the reactor.

"(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 Indian Express stated IAEA as saying.

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Meanwhile, the IAEA did not have 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.

The Yongbyon complex also produces highly enriched uranium, the other key nuclear fuel. The IAEA report said “there were indications, for a period of time, that the reported centrifuge enrichment facility was not in operation” though regular vehicular movements were observed.

In early 2019, Kim Jong Un, the North Korean leader offered to dismantle the entire complex if he won extensive sanctions relief during a summit with then-President Donald Trump. But the Americans rejected Kim's offer because it would only be a partial surrender of his nuclear capability.

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North Korea is believed to be running multiple other covet uranium enrichment facilities. According to a South Korean estimate in 2018, North Korea might already have manufactured 20-60 nuclear weapons as well.

South Korea’s Unification Ministry spokesperson Lee Jong-joo said on Monday that South Korea was closely monitoring North Korea's nuclear and missile activities along with the United States.