Staff at Ukraine's Zaporizhzhia Nuclear Power Plant continue to operate it, but management is now under the orders of the commander of the Russian forces that seized it last week, the U.N. nuclear watchdog said on Sunday, citing Ukraine's regulator.

"I'm extremely concerned," International Atomic Energy Agency chief Rafael Grossi said in a statement, adding Russian forces had cut off some mobile networks and the internet, complicating communications with the plant.

"Management and staff must be allowed to carry out their vital duties in stable conditions without undue external interference or pressure."