Satellite images show a high level of activity at North Korea’s main nuclear site, a U.S. think tank reported on Saturday after the North Korean leader ordered an increase in production of bomb fuel to expand the country’s nuclear arsenal.

The Washington-based 38 North North Korea monitoring project said the activity it had spotted, based on images from March 3 and 17, could indicate that an Experimental Light Water Reactor (ELWR) at the Yongbyon site was nearing completion and transition to operational status, Reuters reported.

The report said the images showed that a 5 megawatt reactor at Yongbyon continued to operate and that construction had started on a support building around the ELWR. Further, water discharges had been detected from that reactor’s cooling system. New construction had also started around Yongbyon’s uranium enrichment plant, likely to expand its capabilities.

“These developments seem to reflect Kim Jong Un’s recent directive to increase the country’s fissile material production to expand its nuclear weapons arsenal,” the report added, referring to the North Korean leader.

On Tuesday, North Korea unveiled new, smaller nuclear warheads and vowed to produce more weapons-grade nuclear material to expand its arsenal, while denouncing stepped up military exercises by South Korea and the United States.

Its state media said Kim had ordered the production of weapons-grade materials in a “far-sighted way” to boost the country’s nuclear arsenal “exponentially.”

It is unclear whether North Korea has fully developed miniaturized nuclear warheads needed to fit on smaller weapons it has displayed and analysts say perfecting such warheads would most likely be a key goal if it resumes nuclear testing for the first time since 2017.

South Korea and the United States have warned since early 2022 that North Korea may resume nuclear testing at any time.

In a report last year, the Stockholm International Peace Research Institute (SIPRI) estimated North Korea had assembled up to 20 nuclear warheads, and probably possessed sufficient fissile material for approximately 45–55 nuclear devices.