The UN atomic watchdog has said North Korea appears to have restarted a nuclear reactor that is widely believed to have produced plutonium for nuclear weapons.

The International Atomic Energy Agency has had no access to North Korea since Pyongyang expelled its inspectors in 2009. The country then pressed ahead with its nuclear weapons program and soon resumed nuclear testing. Its last nuclear test was in 2017.

The IAEA now monitors North Korea from afar, largely through satellite imagery.

In an annual report, the UN nuclear watchdog said "There were no indications of reactor operation from early December 2018 to the beginning of July 2021" the report said of the 5-megawatt reactor at Yongbyon, a nuclear complex at the heart of North Korea's nuclear program.

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

The IAEA issues the report annually before a meeting of its member states, posting it online with no announcement. The report was dated Friday.

In June, the IAEA said there were indications at Yongbyon of possible reprocessing work to separate plutonium from spent reactor fuel that could be used in nuclear weapons. 

The latest report said the duration of that apparent work five months, from mid-February to early July - suggested a full batch of spent fuel was handled, in contrast to the shorter time needed for waste treatment or maintenance.

"The new indications of the operation of the 5MW(e) reactor and the Radiochemical (reprocessing) Laboratory are deeply troubling" it said.

There were indications "for a period of time" that what is suspected to be a uranium enrichment plant at Yongbyon was not in operation, it said. There were also indications of mining and concentration activities at a uranium mine and plant at Pyongsan, it added.

North Korea says its nuclear weapons program is for defensive purposes. It has repeatedly opposed joint annual war games between South Korea and the United States. Pyongyang views the war games as rehearsals for an invasion. 

This year, the North called on Seoul to end the military drills saying they are complicating peace efforts between the two sides. Nevertheless, the South went ahead with the military exercises.