Both sides accused each other of trying to sabotage the visit by the International Atomic Energy Agency (IAEA) to the Zaporizhzhia nuclear plant in southern central Ukraine, which is controlled by Russian forces but operated by Ukrainian staff.
A Reuters reporter in Russian-controlled Enerhodar near the plant said a residential building was struck by shelling, forcing people to take cover in a basement.Â
Conditions at the nuclear plant, Europe's largest, have been unravelling for weeks, with Moscow and Kyiv regularly trading blame for shelling in the vicinity and fuelling fears of a Chornobyl-style radiation disaster.
Earlier, the chief of the IAEA told reporters in the city of Zaporizhzhia, some 55km from the plant, he was aware of "increased military activity in the area" but would press ahead with the plan to visit the facility and meet staff.
"Having come so far, we are not stopping," said Rafael Grossi.
Shortly afterwards, operator Energoatom announced the shutdown of one of only two reactors at the plant.
Meanwhile, Russia accused Ukrainian forces of trying to seize the plant and also of shelling both the meeting point of the IAEA delegation and the nuclear plant itself.
Russia's defence ministry said in a statement that up to 60 Ukrainian troops had crossed the Dnipro river, which divides territory held by the two sides, in boats at 6am on Thursday, in what it said was a "provocation" aimed at disrupting the planned IAEA visit.
The ministry said that "measures had been taken" to destroy the opposing troops, including use of military aviation.
Separately, a local Russian-installed official, Vladimir Rogov, told state broadcaster RT that Ukrainian forces launched an attack out of "desperation" over the IAEA's inspectors' visit. He said Ukrainian assault troops were now pinned down by Russian air force.
Russian state news agency TASS reported that residential areas in the town of Enerhodar had come under "massive" shelling from Ukrainian troops overnight, citing Russian-appointed authorities.
Grossi said on Wednesday the IAEA mission aimed "to prevent a nuclear accident".
Russian-installed officials have suggested that the team from the UN nuclear watchdog would have only a day to inspect the plant, while the mission is preparing 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 said.
Oleksandr Starukh, the head of the Zaporizhzhia region, said Russian troops had shelled the route of the IAEA mission planned to use to reach the power station.
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 President Volodymyr Zelenskiy, referring to the Ukrainian offensive.
Moscow has denied reports of Ukrainian progress and said its troops had routed Ukrainian forces.
Russia captured large tracts of southern Ukraine close to the Black Sea coast in the early weeks of the over six-month-old war, including in the Kherson region, north of the Russian-annexed Crimean Peninsula.
Elsewhere, Ukraine repelled Russian attacks in the direction of Bakhmut and Avdiivka, towns north of the Russian-occupied city of Donetsk, its armed forces' general staff said.
Pro-Russian troops have focused on Bakhmut in their push to extend control over the Donbas region, Ukraine's industrial heartland in its east, the general staff added on Wednesday.
Russian-backed separatists said on Thursday that 13 emergency service personnel were killed and nine wounded after coming under Ukrainian artillery fire in the Russian-controlled part of Donetsk region in eastern Ukraine.
