Washington: North Korea has continued to make rapid improvements to its major nuclear facility in Yongbyon, although the action may have nothing to do with its recent pledge to denuclearize, a US website has said.

38 North, which monitors developments inside the reclusive regime, said the improvements to infrastructure at Yongbyon Nuclear Scientific Research Centre were visible in commercial satellite imagery taken last Thursday, Yonhap news agency reported on Tuesday.

The cooling system for Yongbyon's 5 MWe plutonium production reactor has undergone modifications, although it's unclear whether the reactor is in operation, the website said.

At the Radiochemical Laboratory, used to separate plutonium from spent fuel rods, "a small non-industrial building of an unknown purpose" has been erected.

"Other construction activities were apparent throughout the nuclear complex, including at the Experimental Light Water Reactor, where a new engineering office building looks externally complete and an additional small building has been erected," 38 North said.

"Continued work at the Yongbyon facility should not be seen as having any relationship to North Korea's pledge to denuclearize," the website said.

"The North's nuclear cadre can be expected to proceed with business as usual until specific orders are issued from Pyongyang."

North Korean leader Kim Jong-un committed to the "complete denuclearization" of the Korean Peninsula when he met US President Donald Trump in Singapore on June 12.

Yongbyon, comprising more than 600 buildings, is the centre of the North Korean nuclear programme and its closure and dismantling would be essential for Pyongyang to fulfil its commitment of denuclearization.