Moscow accused Ukrainian forces of trying to seize the plant hours before the IAEA team was due to arrive.
Russia’s defence ministry said up to 60 Ukrainian troops had crossed the Dnipro river, which divides territory held by the two sides, in boats at 6am local time, in what it said was a “provocation” aimed at disrupting the IAEA visit.
The ministry said “measures had been taken” to destroy the opposing troops, including use of military aviation.
A local Russian-installed official, Vladimir Rogov, later said “around 40” of the 60 Ukrainian troops had been killed.
Russian troops also captured three Ukrainian servicemen during the assault on the plant, he added.
Ukrainian officials have welcomed the IAEA visit, expressing hope that it will lead to the demilitarisation of the plant.
They say Russia has been using the plant as a shield to hit towns, knowing it will be hard for Kyiv’s forces to return fire.
They have also accused Russian forces of shelling the plant, which Russian officials deny.
Reuters journalists who followed the IAEA convoy before being ordered to turn back due to the dangerous conditions said that while they were in the city of Zaporizhzhia during the night, they had seen flashes of explosions in the sky.
They could not verify who was responsible.
Russian-installed officials have suggested that the team from the U.N. nuclear watchdog would have only a day to inspect the plant, while the mission had prepared for longer.
“If we are able to establish a permanent presence, or a continued presence, then it’s going to be prolonged. But this first segment is going to take a few days,” Grossi had said.
Both sides have claimed battlefield successes amid a new Ukrainian push to recapture territory in the south.
“It is a very slow process, because we value people,” said Oleksiy Arestovych, an adviser to Zelensky, referring to the Ukrainian offensive.
Moscow has denied reports of Ukrainian progress and said its troops had routed Ukrainian forces.
Ukraine’s southern military command said it would not immediately name settlements in the south it had recaptured to avoid prompting Russian strikes on them.
It also said its counter-offensive was not affecting a Black Sea corridor created to allow for exports of Ukrainian grain.
Russia captured large tracts of southern Ukraine close to the Black Sea coast soon after launching its invasion on Feb 24, including in the Kherson region, north of the Russian-annexed Crimean Peninsula.
In eastern Ukraine, Ukrainian forces repelled Russian attacks in the direction of Bakhmut and Avdiivka, towns north of the Russian-occupied city of Donetsk in Ukraine’s industrial Donbas region, Kyiv’s military general staff said.
Russian-backed separatists said 13 emergency service personnel were killed and nine wounded after coming under Ukrainian artillery fire in the Russian-controlled part of the Donetsk region.
Reuters was not able to independently verify the reports.
Russia calls its actions in Ukraine a “special military operation” to rid Ukraine of nationalists and protect Russian-speaking communities.
Ukraine and the West describe Russia’s actions as an unprovoked war of aggression that has caused millions to flee, killed thousands and turned cities into rubble.