Ukraine on Wednesday accused Russia of carrying out rocket strikes that killed 13 civilians in areas near a nuclear power plant, as the G7 warned that Russian control of the facility "endangers the region".

The overnight strikes in the Dnipropetrovsk region in central Ukraine also injured 11 people, with five reported to be in a serious condition.

"It was a terrible night," regional governor Valentin Reznichenko wrote on Telegram, urging residents to shelter when they hear air raid sirens.

"I am asking and begging you... Don't let the Russians kill you," he wrote, adding that Russia had fired a total of 80 rockets at the area.

Most of the casualties were in the town of Marganets, just across the Dnieper River from the Zaporizhzhia nuclear power plant, Europe's biggest.

Regional council head Mykola Lukashuk said the strikes had hit a local power line, leaving thousands of people without electricity.

G7 call over nuclear plant

Ukraine and Russia have accused each other of recent shelling around the plant, which has six reactors.

Ukraine says Russia has stationed hundreds of troops and stored ammunition at the facility since taking it over on March 4, shortly after starting its invasion.

The tensions have brought back memories of the 1986 Chernobyl nuclear disaster in then Soviet Ukraine, which killed hundreds of people and spread radioactive contamination over much of Europe.

The Group of Seven industrialized nations condemned Russia's occupation and called on Moscow to immediately hand back full control of the plant.

Ukrainian staff operating the plant "must be able to carry out their duties without threats or pressure. It is Russia's continued control of the plant that endangers the region," the G7 foreign ministers said in a statement.

The strikes came a day after major blasts at the Saki airfield, a key military base on the Russian-annexed Crimea peninsula.

Moscow insisted that the explosions were caused by detonating ammunition rather than Ukrainian fire and Ukraine has not claimed responsibility.

'There is a lot of shooting'

Fighting also ground on in eastern Ukraine, where Russian troops are gradually advancing.

The city of Soledar was under constant shelling, AFP reporters saw, as Russian troops attempted to drive out Ukrainian forces and seize a foothold towards the larger city of Bakhmut.

The echoes of cluster bombs and artillery bounced off apartment buildings with their windows shattered, while roads were cratered and shops boarded up or destroyed.

The city was shrouded with black and white smoke arising from artillery and air strikes.

Some of those who remain now live their lives underground in cellars ill-suited as bomb shelters.

"Most have left. It's very scary. There is a lot of shooting," said 62-year-old Svitlana Klymenko.

"I just want to leave to grow old in a normal way, die a normal death, not be killed by a missile."

Vienna dims street lights

The war has severely hampered grain supply from Ukraine, leading to an international food crisis as it is one of the world's biggest producers.

Some ships have been able to leave Ukrainian ports in recent days after a deal with Russia brokered by the United Nations and Turkey.

The first grain shipment to leave on the Sierra Leone-flagged vessel Razoni departed the Ukrainian port of Odessa on August 1 and had been expected to dock in the Lebanese port of Tripoli at the weekend.

But the Ukrainian embassy said a new buyer for the shipment was being sought after the original Lebanese buyer cancelled the order.

A five-month delay after Russia's invasion "prompted the buyer and the shipping agent to reach agreement on the cancellation of the order," the Ukraine embassy said in a statement late Tuesday.

It is currently anchored off the Turkish port of Mersin.

Western countries have meanwhile imposed increasingly stringent sanctions on Russia, raising fears that Russia may cut off gas supplies.

EU countries have started putting into place different measures to save energy, with air conditioning curbs coming into force in Spain on Wednesday and Vienna dimming street lighting.