Recent satellite images show North Korea is expanding a uranium enrichment plant at its main nuclear complex, in a sign it is intent on boosting production of bomb materials, experts have said.

The assessment came after North Korea recently raised tensions by performing its first missile tests in six months amid long-dormant nuclear disarmament diplomacy with the US.

“The expansion of the enrichment plant probably indicates that North Korea plans to increase its production of weapons-grade uranium at the Yongbyon site by as much as 25%,” Jeffrey Lewis and two other experts at Middlebury Institute of International Studies at Monterey said in a report.

The report said satellite images taken by Maxar show construction in an area adjoining the uranium enrichment plant at Yongbyon.

It said a satellite image taken on September 1 shows North Korea cleared trees and prepared the ground for construction and that an excavator is also visible.

The report said a second image taken on September 14 showed a wall erected to enclose the area, work on a foundation and panels removed from the side of the enrichment building to provide access to the newly enclosed space.

“The new area is approximately 1,000 square metres, enough space to house 1,000 additional centrifuges,” the report said. “The addition of 1,000 new centrifuges would increase the plant’s capacity to produce highly enriched uranium by 25%.”

Nuclear weapons can be built using highly enriched uranium or plutonium, and North Korea has facilities to produce both at Yongbyon.

Last month, earlier satellite photos of Yongbyon showed signs that North Korea was resuming the operation of other facilities to produce weapons-grade plutonium.

North Korea calls the complex “the heart” of its nuclear programme. During a summit with then-president Donald Trump in early 2019, North Korean leader Kim Jong Un offered to dismantle the complex if he was given major sanctions relief, but the Americans rejected his proposal because they viewed it as a limited denuclearisation step.

US and South Korean experts speculate Pyongyang is covertly running other uranium-enrichment plants. In 2018, a senior South Korean official told parliament North Korea was estimated to have already manufactured up to 60 nuclear weapons.

Estimates on how many nuclear weapons North Korea can add each year range from six to as many as 18 bombs.

In the past week, North Korea launched ballistic and cruise missiles towards the sea in tests seen as an effort to diversify its missile forces and strengthen its attack capability on South Korea and Japan, where a total of 80,000 American troops are based.

Experts say both types of missiles could be armed with nuclear warheads.

Mr Kim has threatened to bolster his nuclear arsenal and acquire more sophisticated weapons unless Washington drops its hostility against his country, in an apparent reference to US-led sanctions and its regular military drills with Seoul.

But he maintains his self-imposed moratorium on testing long-range missiles directly targeting the US mainland, a suggestion that he wants to keep chances for future diplomacy with Washington alive.