Ukraine said on Sunday that renewed Russian shelling had damaged three radiation sensors and hurt a worker at the Zaporizhzhia power plant, in the second hit in consecutive days on Europe's largest nuclear facility.

Ukrainian President Volodymyr Zelenskiy called Saturday night's shelling "Russian nuclear terror" that warranted more international sanctions, this time on Moscow's nuclear sector.

"There is no such nation in the world that could feel safe when a terrorist state fires at a nuclear plant," Zelenskiy said in a televised address on Sunday.

However, the Russian-installed authority of the area said it was Ukraine that hit the site with a multiple rocket launcher, damaging administrative buildings and an area near a storage facility.

Reuters could not verify either side's version.

Events at the Zaporizhzhia site - where Kyiv had previously alleged that Russia hit a power line on Friday - have alarmed the world.

"(It) underlines the very real risk of a nuclear disaster," International Atomic Energy Agency head Rafael Mariano Grossi warned on Saturday.

Elsewhere, a deal to unblock Ukraine's food exports and ease global shortages gathered pace as another four ships sailed out of Ukrainian Black Sea ports while the first cargo vessel since Russia's Feb. 24 invasion docked. 

The four outgoing ships had almost 170,000 tonnes of corn and other food. They were sailing under a deal brokered by the United Nations and Turkey to try to help ease soaring global food prices that have resulted from the war.

Before Moscow's Feb. 24 invasion, which Russian President Vladimir Putin calls a "special military operation", Russia and Ukraine together accounted for nearly a third of global wheat exports. The disruption since then has threatened famine in some parts of the world.

BATTLE FOR DONBAS

Putin's troops are trying to gain full control of the Donbas region of east Ukraine where pro-Moscow separatists seized territory after the Kremlin annexed Crimea to the south in 2014.

Russian forces stepped up their attacks north and northwest of Donetsk city in the Donbas on Sunday, Ukraine's military said. The Russians attacked Ukrainian positions near the heavily fortified settlements of Piski and Avdiivka, as well as shelling other locations in the Donetsk region, it said.

In addition to tightening its grip over the Donbas, Russia is entrenching its position in southern Ukraine, where it has gathered troops in a bid to prevent a potential counter-offensive near Kherson, Kyiv has said.

As the fighting rages, Russians installed in the wake of Moscow's invasion have toyed with the idea of joining Ukraine's occupied territory to Russia. Last month, a senior pro-Russian official said a referendum on such a move was likely "towards next year." 

In his video address, Zelenskiy said that any "pseudo-referendums" on occupied areas of his country joining Russia would eliminate the possibility of talks between Moscow and its Ukrainian counterparts or their allies.

"They will close for themselves any change of talks with Ukraine and the free world which the Russian side will clearly need at some point," Zelenskiy said. 

Also Sunday, Ukraine's chief war crimes prosecutor said almost 26,000 suspected war crimes committed since the invasion were being investigated, with 135 people charged, of whom 15 were in custody. Russia denies targeting civilians.

Shelling and missile strikes were reported overnight in the Ukrainian city of Kharkiv and around military sites in the western region of Vinnitsya, among other places, Ukrainian authorities said. There was no immediate word on casualties.

Beyond Ukraine, a proxy battle played out at the International Chess Federation where former Russian deputy prime minister Arkady Dvorkovich won a second term as president, defeating Ukraine's Andrii Baryshpolets. 

And after days of controversy, Amnesty International apologised for "distress and anger" caused by a report accusing Ukraine of endangering civilians. That had infuriated Zelenskiy and prompted the head of the rights group's Ukraine office to resign.