Recent satellite images of North Korea's main nuclear complex suggest continued activity focused on the production of both weapons-grade plutonium and uranium, a US think-tank said Thursday.

The June 30 images of the Yongbyon complex show water being discharged from its ageing five megawatt reactor -- a product of the secondary cooling system, the Institute for Science and International Security (ISIS) said in a report.

"However, without more data, such as regular steam production, it is hard to determine the operational status of the reactor and thus to estimate the amount of plutonium produced," the report said.

The reactor, shut down in 2007 under an aid-for-disarmament accord, is capable of producing six kilograms (13 pounds) of plutonium a year -- enough for one nuclear bomb.

North Korea began renovating the facility after its last nuclear test in 2013, and previous satellite images suggested it became operational in October that year.

The latest imagery also showed continued construction at the complex's gas centrifuge plant.

The North 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 ISIS 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.