North Korea has started building an annex onto its original uranium enrichment plant at Yongbyon, according to satellite imagery analysed by NK Pro.

Planet Labs images show work began in mid-September and had not started by September 7, the Seoul-based specialist service reported on September 29. A Maxar image taken on September 28 shows an L-shaped annex of about 1,300 square metres going up on the south-east corner of the old building, with blue roofing already laid over most of it - a sign, in NK Pro's reading, that the authorities are in a hurry.

The extension adds to capacity that is already growing on several fronts. In a report dated August 28 the International Atomic Energy Agency (IAEA) identified a second, two-storey enrichment plant at Yongbyon with room for up to 28 centrifuge cascades. By NK Pro's count, Pyongyang has now enlarged its enrichment sites three times and built a new one in the past five years.

The purpose of the new work is not yet clear. Its shape and footprint match an annex added to the same building in 2021 and another built in 2024 at Kangson, a second enrichment site near Pyongyang. State media published the first photographs of Kim Jong Un walking among rows of centrifuges at Kangson in September 2024, and at Yongbyon in January 2025.

The Kangson annex shows how long such a hall takes to fill. The IAEA believes centrifuge cascades inside it started operating in January, about 22 months after construction began, according to the IAEA. The comparison suggests a similar lag for the new Yongbyon annex, though its operating date is unknown.

IAEA director general Rafael Grossi warned in Seoul on April 15 that the expansion amounted to "a very serious increase in the capabilities of (the) DPRK in the area of nuclear weapons production, which is estimated at a few dozen warheads", Euronews reported.

The Stockholm International Peace Research Institute (SIPRI) estimates in its SIPRI Yearbook 2026 that as of January North Korea had probably assembled around 60 warheads and held enough fissile material for at least 90.

Kim has made clear he wants more. During an apparent visit to Yongbyon in June he ordered "ambitious future plans designed to strengthen our state's nuclear forces exponentially", state media reported. He called for "accelerating" nuclear weapons development at a party plenum the same month, and for "bolstering up the nuclear force both in quality and quantity" at a military meeting in July.

The nuclear build-up is running alongside a conventional one. This month North Korea for the first time drilled swarm attacks mixing drones and missiles against a single target, and in May it wrote automatic nuclear retaliation into its constitution should its leadership be attacked, according to South Korean intelligence.