North Korea has built a new two-storey uranium enrichment facility, the head of the world’s nuclear watchdog said this week, calling it a cause for “serious concern”.

The new building is part of the main Yongbyon nuclear complex, and reportedly capable of housing up to 28 centrifuge cascades used to enrich uranium.

In a report published late last month, the International Atomic Energy Agency said Pyongyang has also begun operating an expanded annex at another enrichment site.

These findings point to a steady expansion of the North’s sancitoned nuclear infrastructure. The agency, which has no inspectors in North Korea since it was expelled in 2009, relies on satellite imagery for assessments.

“The continuation and further development of the DPRK’s nuclear programme are clear violations of relevant UN Security Council resolutions and are deeply regrettable,” IAEA director Rafael Grossi said in a statement earlier this week.

The IAEA first referred to the existence of the building in June, although Grossi had mentioned the possibility earlier in the year.

In June, the agency said that North Korean leader Kim Jong Un visited a “newly inaugurated nuclear materials production factory” on 3 June, which it identified as the Yongbyon enrichment facility.

Kim had praised the country’s “greatly expanded production capacity”, adding that this would “offer the possibility of carrying out larger plans”, according to the agency’s report.

Despite running one the poorest countries in the world, Kim spends millions each year developing a secretive nuclear arsenal. A 2022 report by a non-proliferation think-tank put this figure at $640m.

“What makes North Korea’s nuclear programme so concerning is that many of its facilities are unknown, concealed, and thus difficult to target,” Edward Howell, a lecturer at the University of Oxford told the BBC.

“Beyond Yongbyon, we know that other facilities exist: Kangson, for instance. The embeddedness of North Korea’s nuclear doctrine within its foreign and domestic policy means that we should expect there to be more concealed production sites.”

The agency also said there were indications that operations had started at another enrichment facility in Kangson, near the capital Pyongyang, in January this year. An annex had first been constructed in Kangson in 2024, it said.

It had said in April that North Korea appeared to be stepping up its ability to make nuclear bombs, saying they were monitoring the apparent expansion of Yongbyon.

The North’s nuclear programme has accelerated under Kim, who has overseen a record number of missile tests. The country has fired has numerous intercontinental ballistic missiles (ICBMs) since it began testing its long-range system in 2017.

The country has conducted six nuclear weapons tests so far, the last of which was in September 2017.