Nuclear-armed North Korea has started operating a new plant inside its uranium enrichment complex, boosting its capacity to generate fuel that could be used to make atomic weapons, a report said Wednesday.

"Infrared cameras used by South Korean and US intelligence authorities have detected heat that was emitted when gas centrifuges in the new plant began operating," the Joongang Ilbo daily quoted an unnamed official in charge of handling North Korean intelligence.

The North started building the new facility inside the Yongbyon nuclear complex in 2012 and began operating it recently after completing construction, the report said.

"We have to monitor a little longer to see if the new plant started producing (enriched uranium) at full blast but it is our assessment that it is running," the newspaper cited the source as saying.

A defence ministry official told AFP it could not comment on "any matters of intelligence".

In August, the Institute for Science and International Security (ISIS) said satellite images of Yongbyon suggested continued production at the centrifuge plant of the North's main nuclear complex.

Pyongyang says the plant is dedicated to producing low-enriched uranium for an under-construction Light Water Reactor (LWR), but experts suspect that the final goal is weapons-grade uranium.

Previous imagery showed the centrifuge building had doubled in size, and the US think tank said it was likely that this year had seen the installation of centrifuge cascades inside the new section.

Overall, the latest images, combined with procurement data obtained by ISIS, suggest that North Korea "is emphasising the production of weapon-grade plutonium as well as enriched uranium for its nuclear weapons program," the report said.

Pyongyang is currently believed to have enough plutonium for about six bombs, after using part of its stock for at least two of its three atomic tests.

ISIS has estimated that the expanded centrifuge plant could produce as much as 68 kilograms of weapons-grade uranium a year -- enough for three nuclear bombs with a little left over.