N Korea has built two-storey uranium enrichment facility, says watchdog

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.