The International Atomic Energy Agency (IAEA) has repeatedly raised concern over the safety of the six-reactor plant in southern Ukraine, which has been on the front line since Russian forces seized it last year.

Blaming stepped-up shelling from the Ukrainian side, Russia last week ordered families with children and elderly to temporarily leave the nearby town of Enerhodar.

"The general situation in the area near the Zaporizhzhya Nuclear Power Plant is becoming increasingly unpredictable and potentially dangerous," IAEA chief Rafael Grossi said in a statement on Saturday. "I'm extremely concerned about the very real nuclear safety and security risks facing the plant."

While the nuclear plant's staff remain onsite, IAEA experts have "received information that the announced evacuation of residents from the nearby town of Enerhodar -- where most plant staff live -- has started and they are closely monitoring the situation for any potential impact on nuclear safety and security," Grossi added.

The Enerhodar evacuation is part of a wider removal order that extends to 18 other frontline locations in the Zaporizhzhia region.

Ivan Fedorov, mayor of Melitopol, a city in Zaporizhzhia Oblast, accused the Russian forces of proceeding too quickly with the evacuation, with buses departing every 20-30 minutes starting Friday.

Posting pictures on Telegram of the checkpoint between Russian-occupied Melitopol and annexed Crimea, he said on Saturday that there was "a mad panic and no less mad queues".

"The partial evacuation they announced is going too fast, and there is a possibility that they may be preparing for provocations and (for that reason) focusing on civilians," Fedorov wrote on Telegram.

Moscow has blamed Kyiv -- and its Western supporters -- for a series of attacks and sabotage operations, including on the Kremlin.

Ukraine has been preparing an offensive against Russian forces for months and some analysts see these recent incidents as a sign it is now imminent.