Britain's defense ministry says Russian forces have likely established fighting positions at the Zaporizhzhia nuclear power plant in southern Ukraine.

The ministry said on Thursday that satellite imagery shows that by March, "Russian forces had established sandbag fighting positions on the roofs of several of the six reactor buildings" at the nuclear plant.

It said Russia has likely constructed these positions because it is increasingly concerned about a possible major Ukrainian offensive.

The ministry also said, "This is the first indication of the actual reactor buildings being integrated in tactical defense planning."

It expressed concern that the move is highly likely to increase the chances of damage to safety systems if fighting takes place around the plant.

Ukrainian President Volodymyr Zelenskyy said on social media on Thursday that Russia had attacked the southern region of Mykolaiv at night with missiles from the Black Sea. He said the attacks killed at least one civilian and wounded 23 others, and that a high-rise building and private houses were destroyed.

Zelenskyy also said he held phone talks with IAEA Director General Rafael Mariano Grossi on Wednesday, the 37th anniversary of the nuclear disaster at the Chornobyl plant.

The president stressed that only the return of Ukraine's full control over the Zaporizhzhia plant will protect the world from a new disaster.