Russia on Friday ordered the evacuation of families with children and of the elderly from Russian-held frontline areas in southern Ukraine because of an increase in shelling from the Ukrainian side.

"In the past few days, the enemy has stepped up shelling of settlements close to the front line," the Russian-installed head of the Zaporizhzhia region, Yevgeny Balitsky, wrote on social media.

"I have therefore made a decision to evacuate first of all children and parents, elderly people, disabled people and hospital patients," he wrote.

"There will be a temporary evacuation" from 18 places, he said, including the town of Energodar where the Zaporizhzhia nuclear power plant is located.

The plant -- Europe's biggest -- has been held by Russia since the first day of the conflict and is now near the front line, causing international concern.

Ukraine has been preparing an offensive against Russian forces for months and some analysts say sabotage attacks and long-distance strikes behind Russian lines in recent days show it is now imminent.