Diplomatically isolated North Korea is believed to operate multiple facilities for enriching uranium, a key step in making nuclear warheads, South Korea's spy agency has said.

That includes one at the Yongbyon nuclear site, which Pyongyang purportedly decommissioned after talks but later reactivated in 2021.

"In our periodic assessments, we have been able to confirm that there's a rapid increase in the operations" of the Yongbyon reactor, International Atomic Energy Agency (IAEA) chief Rafael Grossi said in Seoul, where he met South Korea's foreign minister.

The agency also observed a rise in operations at Yongbyon's reprocessing unit and light-water reactor, as well as the activation of other facilities, Grossi told reporters.

"All that points to a very serious increase in the capabilities of (the) DPRK in the area of nuclear weapons production, which is estimated at a few dozen warheads," he said, referring to the Democratic People's Republic of Korea, the North's official name.

North Korea, which conducted its first nuclear test in 2006, is under rafts of UN sanctions for its banned weapons programmes.

It has declared that it will never surrender its nuclear weapons, and cut off access to IAEA inspectors in 2009.

The agency has noted the construction of a "new facility similar to the enrichment facility in Yongbyon", Grossi said.

It was "not easy to calculate" any production increases without visiting the site.

However, "we consider, looking at external features of the facility, that there will be significant increase in the enrichment capacity of the DPRK", he said.

The Center for Strategic and International Studies said this week that North Korea appeared to have completed a building at Yongbyon that could be a new uranium enrichment plant.

Citing satellite imagery from April, the US-based think tank said the building had generators, fuel storage tanks and cooling units.

- Russia connection unclear -

Asked whether Russia was assisting North Korea's nuclear development, Grossi said the IAEA had not seen "anything in particular in that regard".

While the agency hoped any such cooperation would be civilian in nature, "if anything, this is too early days to judge", he said.

North Korea has sent ground troops and artillery shells to support Russia's invasion of Ukraine, and observers say Pyongyang is receiving military technology assistance from Moscow in return.

Grossi told South Korean Foreign Minister Cho Hyun that Pyongyang's nuclear programme "remained one of the IAEA's key issues", a statement from the ministry said later on Wednesday.

Cho said Seoul was working to "end hostility and confrontation" with the North and to pursue peaceful coexistence and shared growth on the peninsula.

Separately, top naval commanders from South Korea, the United States and Japan met in Seoul on Wednesday to hold maritime security talks aimed at deterring North Korea's growing nuclear and missile threats, Seoul's navy said.