Russia said the Zaporizhzhia nuclear power plant in southern Ukraine would operate under Moscow's supervision.
The announcement came after the Kremlin had declared the annexation of the Zaporizhzhia region along with three others this week.
The Ukrainian director of the nuclear plant - Europe's biggest - was last week briefly detained by the Russians, and the UN nuclear watchdog says he will not return to his job.
But the head of Ukraine's state nuclear energy regulator Enerhoatom, Petro Kotin, has just announced he will be taking over running the plant from the capital Kyiv.
In a video statement, he urged the station's Ukrainian personnel not to sign any contracts with Russian "occupiers".
It is not clear how Kotin will be able to run the facility, as it is located in the Russian-held part of the Zaporizhzhia region.
Russian troops seized the plant in the first days of their invasion launched on 24 February.
