North Korea appears to have restarted activities at its main plutonium-producing reactor in Yongbyon, the UN atomic agency, IAEA, has said in an annual report.

The apparent resumption of operations comes as Pyongyang has been voicing threats to expand its nuclear arsenal amid continued tensions with the United States.

What did the IAEA say in its report?

The IAEA has not had access to this or other nuclear facilities in the country since its inspectors were expelled in 2009. It says it monitors nuclear activities in North Korea largely through satellite imagery.

There had been no sign of reactor operation from "December 2018 to the beginning of July 2021." 

However, since early July 2021, "there have been indications, including the discharge of cooling water, consistent with the operation of the reactor."

The report also said that a radiochemical laboratory at the Yongbyon site seemed to have operated from mid-February to early July this year. The laboratory extracts plutonium from spent fuel rods.

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 in the report, dated Friday.

Why is the Yongbyon reactor important?

The Yongbyon reactor produces plutonium — a key ingredient for building nuclear weapons along with highly enriched uranium.

North Korea has described the reactor as being "the heart" of its nuclear program and research. 

The complex has been the focus of international concern about the communist country's nuclear weapons program.

North Korea has recently threatened to step up its nuclear weapons program if the United States did not step back from its "hostile" policy toward it. This appears to refer to US-led sanctions and regular joint military drills carried out by Washington and Seoul.

North Korea's last nuclear test was in 2017.

tj/rt (AP, Reuters)