This satellite image, released by 38 North on Nov. 24, 2021, shows North Korea's Yongbyon nuclear complex, north of Pyongyang. Yonhap

The chief of the U.N. nuclear watchdog said Monday there are signs that North Korea is operating the Yongbyon nuclear complex's uranium enrichment facility while carrying out construction work there.

Rafael Grossi, Director General of the International Atomic Energy Agency (IAEA), made the remarks amid speculation that North Korea appears to be preparing for what would be its first nuclear test since 2017.

"We have observed indications that the reported centrifuge enrichment facility at Yongbyon continues to operate and is now externally complete, expanding the building's available floor space by approximately one-third," Grossi said in a statement to the board of governors.

He said there are indications that the 5MW reactor is operating at the Yongbyon site and intermittent activity at the radiochemical laboratory has been observed.

Grossi also said there are "ongoing indications of activities" at the Kangson complex and Pyongsan uranium mine and concentration plant.

"The continuation of the DPRK's nuclear program is a clear violation of relevant UN Security Council resolutions and is deeply regrettable," Grossi said, referring to North Korea's official name, the Democratic People's Republic of Korea.

North Korea conducted its sixth and last nuclear test in September 2017. Nuclear talks between the U.S. and North Korea have remained stalled since the 2019 Hanoi summit between the two nations ended without a deal. (Yonhap)