The head of the U.N.’s nuclear watchdog expressed “serious concern” about North Korea’s operation of a new uranium enrichment facility, days after Pyongyang confirmed its existence for the first time.

Rafael Grossi, director general of the International Atomic Energy Agency (IAEA), told the agency’s Board of Governors on Monday that inspectors have linked a facility shown in photographs of a visit by North Korean leader Kim Jong Un to a new building at the Yongbyon nuclear complex.

The nuclear watchdog chief previously confirmedthat construction of the building’s exterior appeared complete in March, noting similarities to an enrichment facility at