Recent satellite images show North Korea is expanding a uranium enrichment plant at its main Yongbyon nuclear complex, a sign that it’s intent on boosting the production of bomb materials, experts say.

The assessment comes after North Korea recently raised tensions with its first missile tests in six months amid long-dormant nuclear disarmament negotiations with the United States. 

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“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 the photos taken by satellite imagery company Maxar showed construction in an area adjoining the uranium enrichment plant at Yongbyon.

It said a satellite image taken on Sept. 1 showed North Korea cleared trees and prepared the ground for construction, and that a construction excavator was also visible. The report said a second image taken on Sept. 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 area.

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The new area is approximately 1,000 square meters (10,760 square feet), enough space to house 1,000 additional centrifuges, which would increase the plant’s capacity to produce highly enriched uranium by 25%, the report said.

Nuclear weapons can be built using either 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 Yongbyon complex “the heart” of its nuclear program. During a summit with then-President Donald Trump in early 2019, North Korean leader Kim Jong Un offered to dismantle the entire complex if he was given major sanctions relief. But the Americans rejected Kim’s proposal because they viewed it as a limited denuclearization step. 

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Some U.S. and South Korean experts speculate North Korea is covertly running at least one additional uranium-enrichment plant. In 2018, a top South Korean official told parliament that North Korea was estimated to have already manufactured up to 60 nuclear weapons as well.

Estimates on how many nuclear weapons North Korea can add every year vary, ranging from six to as much as 18 bombs.

In the past week, North Korea launched both ballistic and cruise missiles toward the sea in tests seen as an effort to diversity 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. 

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Kim has threatened to bolster his nuclear arsenal and acquire more sophisticated weapons unless Washington drops its hostility against his country, an apparent reference to U.S.-led sanctions and its regular military drills with Seoul. But Kim still maintains his self-imposed moratorium on testing long-range missiles directly targeting the U.S. mainland, suggesting he wants to keep chances for future diplomacy with Washington alive.