North Korea's main plutonium-producing reactor has been shut down for enough time for it to be re-fuelled, the International Atomic Energy Agency said in a recent report, stressing Pyongyang's nuclear activities remain a cause for "serious concern."

The 5-megawatt reactor at the North's Yongbyon nuclear complex showed signs of intermittent operation between mid-August and late November last year, but there have been no indications of operations there since early December, the agency said in the report submitted for its general conference this week. 

Such a period of shutdown is enough "for it to have been de-fuelled and subsequently re-fuelled," the IAEA said in the report, which covers developments since its director general's previous report issued in August 2018.

The reactor has been the main source of weapons-grade plutonium for the North. The communist nation can harvest one nuclear bomb's worth of plutonium if it reprocesses thousands of spent fuel rods extracted from the reactor after about a year of operation.

The latest IAEA report also said there have been signs of use at the centrifuge enrichment facility located in Yongbyon, though no indications of reprocessing activities were detected at the radiochemical lab in the plant.

Mining, milling and concentration activities also appeared to have continued at "locations previously declared as the Pyongsan uranium mine and the Pyongsan uranium concentration plant," it said.

The agency noted that it did not have access to Yongbyon and other facilities and that it cannot confirm the operational status or the purpose of the cited activities without such access. The IAEA used other information, such as satellite imagery, to monitor the North's nuclear program.

"The continuation of the DPRK's nuclear programme is a clear violation of relevant UN Security Council resolutions and is deeply regrettable," the report said, urging Pyongyang to comply with its obligations under the resolutions. DPRK stands for the North's official name, the Democratic People's Republic of Korea.

Cornel Feruta, the acting director general of the IAEA, said in a statement that the agency is ready to "play an essential role" in verifying the North's nuclear program "if a political agreement is reached among countries concerned." (Yonhap)