The world narrowly dodged a radiation disaster when electricity to Europe's largest nuclear power plant was cut for hours, Ukrainian President Volodymyr Zelenskyy said, urging international bodies to act faster to force Russian troops to vacate the site.

Zelenskyy said Russian shelling on Thursday sparked fires in the ash pits of a nearby coal power station that disconnected the reactor complex, Europe's largest such facility, from the power grid. A Russian official said Ukraine was to blame.

Back-up diesel generators ensured power supply that is vital for cooling and safety systems at the plant, Zelenskyy said, praising the Ukrainian technicians who operate the plant under the gaze of the Russian military.

"If our station staff had not reacted after the blackout, then we would have already been forced to overcome the consequences of a radiation accident," he said in an evening address.

"Russia has put Ukraine and all Europeans in a situation one step away from a radiation disaster."

A mission from the U.N.'s International Atomic Energy Agency is expected to visit the Zaporizhzhia nuclear power plant next week after it was temporarily knocked offline and more shelling was reported in the area overnight, Ukrainian officials said Friday.

Fire damage to a transmission line at Europe's largest nuclear plant caused a blackout across the region on Thursday and heightened fears of a catastrophe in a country still haunted by the Chernobyl disaster in 1986.

Energoatom said electricity for the plant's own needs was now being supplied through a power line from Ukraine's electricity system, and work was ongoing to restore grid connection to the plant's two functioning reactors.

Vladimir Rogov, a Russian-appointed official in the occupied town of Enerhodar near the plant, blamed Ukraine's armed forces for the incident, saying they caused a fire in a forest near the plant. He said local towns had lost power for several hours.

"This was caused by the disconnection of power lines from the Zaporizhzhia nuclear power station as a result of provocations by Zelenskiy's fighters," Rogov wrote on Telegram. "The disconnection itself was triggered by a fire and short circuit on the power lines."

Russia's Defense Ministry said on Friday its forces had destroyed a U.S.-made M777 howitzer, which it said Ukraine had used to shell the Zaporizhzhia plant. Satellite images showed a fire near the plant, but Reuters could not verify its cause.

Hot spot

Energoatom said it had been the first complete disconnection in the plant, which has become a hot spot in the six-month-old war.

Russia invaded Ukraine in February, captured the plant in March and has controlled it since, though Ukrainian technicians still operate it. Russia and Ukraine have accused each other of shelling the site, fuelling fears of a nuclear disaster.

The United Nations is seeking access to the plant and has called for the area to be demilitarized. International Atomic Energy Agency (IAEA) officials are "very, very close" to being able to visit Zaporizhzhia, agency Director-General Rafael Grossi said on Thursday.

Nuclear experts have warned of the risk of damage to the plant's spent nuclear fuel pools or its reactors. Cuts in power needed to cool the pools could cause a disastrous meltdown.

Paul Bracken, a national security expert and professor at the Yale School of Management, said the concern was that artillery shells or missiles could puncture the reactor walls and spread radiation around potentially a large area, much like the 1986 accident involving the Chornobyl reactor.

A failure at the Zaporizhzhia plant could "kill hundreds or thousands of people, and damage environmentally a far larger area reaching into Europe," Bracken said.

"Russian Roulette is a good metaphor because the Russians are spinning the chamber of the revolver, threatening to blow out the brains of the reactor all over Europe," Bracken said.

Fighting

Russia's ground campaign has stalled in recent months after its troops were repelled from the capital Kyiv in the early weeks of the invasion but fighting continues along the frontlines to the south and east.

Russian forces control territory along Ukraine's Black Sea and Sea of Azov coasts, while the conflict has settled into a war of attrition in the eastern Donbass region, which comprises the provinces of Donetsk and Luhansk.

In its morning roundup of battlefield developments from around the country on Friday, the Ukraine military said its forces had repulsed Russian assaults on the towns of Bakhmut and Soledar in the Donetsk region.

Ukraine's operational command "South" said its artillery had struck ammunition depots and enemy personnel in the southern Kherson region, while airstrikes were launched against enemy air defenses.

Russia's Tass news agency reported that Ukrainian forces using a U.S.-supplied HIMARS multiple rocket launcher targeted the town of Stakhanov in Donbass, with about 10 rockets hitting the town before dawn on Friday, according to pro-Moscow breakaway officials in Luhansk.

Reuters was unable to verify the battlefield reports of either side.

Kyiv has repeatedly called for more, high-grade Western military hardware it says it needs to repel Russian attacks.

Zelenskyy spoke on Thursday by phone with U.S. President Joe Biden, who reiterated U.S. support for Ukraine against Russia, the White House said. Biden announced on Wednesday, Ukraine's independence day, $3 billion of new security assistance for Kyiv, though it could take months or even years to arrive.

Potentially giving additional credence to Western estimates of heavy Russian losses during the war, Russian President Vladimir Putin signed a decree on Thursday increasing the size of Russia's armed forces to 2.04 million from 1.9 million.

The Kremlin says its aim is to "denazify" and demilitarize Ukraine and remove perceived security threats to Russia. Ukraine and the West say this is a baseless pretext for a war of conquest.