Thermal images of North Korea's main nuclear site show Pyongyang may have reprocessed more plutonium than previously thought.

The analysis by 38 North, a Washington-based North Korean monitoring project, is based on satellite images of the radiochemical laboratory at the Yongbyon nuclear plant.

It says if the rogue state has reprocessed more plutonium, it would be able to expand its nuclear weapons programme.

The images will intensify international concerns over North Korea's nuclear and missile programmes.

The think-tank said images of the uranium enrichment facility at Yongbyon could also indicate the operation of centrifuges that could be used to increase North Korea's stock of enriched uranium, another source it uses to fuel its bombs.


Image: A satellite image of the radiochemical lab in Yongbyon

38 North said there were also signs of at least short-term activity at North Korea's Experimental Light Water Reactor that could be cause for concern.

The radiochemical laboratory images showed there had been at least two reprocessing cycles not previously known aimed at producing "an undetermined amount of plutonium that can further increase North Korea's nuclear weapons stockpile".

It is unclear if the thermal activity detected at the uranium plant was the result of centrifuge operations or maintenance.

The think-tank said the thermal patterns at the plant's isotope/tritium production facility suggested it was not operational and was therefore not producing tritium, an essential isotope used in hydrogen weapons.

Celebrating a missile strike - North Korean style

North Korea manufactures its atomic bombs using uranium and plutonium and has tested five nuclear bombs.

Experts fear it could test a sixth at any time, despite US-led international efforts to curb its programme.

Pyongyang said its penultimate test in January 2016 was of a hydrogen bomb, but experts have treated this claim with scepticism.

The country has been working to develop a nuclear-tipped missile capable of hitting the US and last week tested its first intercontinental ballistic missile, which experts said could hit all of Alaska and parts of the US Pacific Northwest.