The UN nuclear watchdog is due to issue a report on the Zaporizhzhia nuclear power station in Ukraine on Tuesday, a day after shelling cut its electricity supplies for the second time in two weeks and raised fears of a catastrophe.

Ukraine and Russia have accused each other of risking nuclear disaster by shelling near Europe's largest nuclear plant, which officials said disrupted power lines on Monday and took the sole remaining reactor offline.

The incident came as Ukrainian forces pressed their counter-attacks in the south and east, raising the national flag over a town in Kherson province, a southern region occupied by Russia since the war's early days.

The International Atomic Energy Agency (IAEA), citing information supplied from Ukraine, said the plant's backup power line had been cut to extinguish a fire but that the line itself was not damaged and would be reconnected.

The UN nuclear watchdog said the plant had enough electricity to operate safely and would be reconnected to the grid once backup power was restored.

The IAEA's presence at the plant was reduced to two staff members from six on Monday. IAEA chief Rafael Grossi will issue a report on Ukraine, including the plant, on Tuesday and then brief the UN Security Council, the IAEA said.

Ukrainian President Volodymyr Zelenskiy on Monday warned of a near "radiation catastrophe" and said the shelling showed Russia "does not care what the IAEA will say."

The nuclear concerns add to the ongoing energy fight between Moscow and the West since Russian troops invaded Ukraine in late February as the larger military conflict continues.

European gas prices soared on Monday as Russia kept its main gas pipeline to Germany shut, bringing fears of a bleak winter for consumers and businesses across the continent.

Moscow blames disruption to equipment repairs and maintenance caused by Western sanctions for its halt to the flow of gas through Nord Stream 1, its main pipeline to Germany. Russia was due to reopen the pipeline on Saturday but is now shut indefinitely.

Europe and the United States accuse Russia of using energy as a weapon and are collaborating to ensure supplies.

Russian Energy Minister Nikolai Shulginov told reporters at the Eastern Economic Forum in Vladivostok on Tuesday that Moscow would respond to proposed price caps on Russian oil by shipping more supply to Asia.

The Kremlin warned the West on Monday that it would retaliate after Group of Seven finance ministers agreed last week to a cap to pressure Russia over its actions in Ukraine.

Fighting

Kyiv on Monday made its boldest claim yet of success on the battlefield in its week-old counter-offensive against Russian forces in the south.

Following days of silence about their new offensive, Ukrainian officials posted an image online of three soldiers raising Ukraine's blue and yellow flag on a rooftop purportedly in Vysokopyllya, in the north of Kherson.

"We will renew our territory. We cannot freeze this conflict now. We need to step-by-step de-occupy our territory," Zelenskiy said in an interview with ABC News. "It's only a matter of time."

Ukraine's southern command said on Tuesday that four Russian ammunition depots had been destroyed in three districts of Kherson region in the previous 24 hours.

Bridges over the Dnipro river had been shelled, it added.

"Control and cover by fire of the crossings of the Dnipro river is systematic and effective," the southern command said in a statement.

Reuters was unable to independently verify the battlefield reports.

In a rare acknowledgement of the Ukrainian counter-offensive, TASS news agency on Monday quoted a Moscow-installed official in the Kherson region as saying plans for a referendum on joining Russia had been put on hold due to the security situation.

Ukraine's general staff late on Monday said Russian forces had been driven back in an unspecified area near Kramatorsk - a key town in the eastern Donetsk region.

On Monday evening, a missile strike by Russian forces destroyed an oil depot in Kryvorizsky district in Dnipropetrovsk region, emergency authorities said.

US President Joe Biden on Monday said Russia should not be designated a state sponsor of terrorism, a label Ukraine has pushed for but which Moscow has warned would rupture US-Russian ties.

- Reuters