Ukraine's nuclear agency says Russian rockets have damaged part of a giant Russian-controlled nuclear power plant, but there has been no radiation leak.
Enerhoatom said a nitrogen-oxygen unit and a high-voltage power line had been damaged at the Zaporizhzhia plant - Europe's largest - in southern Ukraine.
Local Russian-appointed officials blamed Ukraine for shelling earlier.
Ukraine also accuses Russian forces of firing rockets at civilian areas from the site, employing "terror tactics".
"Every morning we wake up and see that they have hit only residential homes," a local businessman told the BBC.
The BBC was unable to verify the reported damage at the nuclear plant. Enerhoatom says there were two rounds of Russian rocket fire on Friday, which prompted the site's operators to disconnect a reactor from the power grid.
Enerhoatom said "there is a risk of hydrogen leakage and dispersal of radioactive particles".
"The fire danger is high. Currently there are no injuries," it added.
Russia seized the Zaporizhzhia plant in March but kept its Ukrainian employees. Russia controls the plant and surrounding areas, close to Ukrainian-held territory. It consists of six pressurised water reactors and stores radioactive waste.
Western officials have sounded the alarm about Russia's tactics there.
The plant is in the city of Enerhodar, in the south-east of Ukraine along the left bank of the River Dnieper (Dnipro in Ukrainian).
The Russian-appointed officials in Enerhodar said Ukrainian forces shelled the plant twice on Friday "from the opposite bank of the Dnieper". "The second time the nationalists managed to hit the target - shells landed in the plant's industrial site," their statement said.
The plant's Moscow-installed management was quoted by Russia's state-run Interfax news agency as saying two of the plant's power lines had been hit by a Ukrainian artillery strike, causing a fire.
It is not clear how many power lines still operate at the plant, and the contrasting claims have not been independently verified.
Earlier, in its daily intelligence update, the UK defence ministry said Russia was using the area to launch attacks - taking advantage of the "protected status" of the nuclear power plant to reduce the risk of overnight attacks from Ukrainian forces.
The head of the UN's nuclear agency, Rafael Grossi, warned this week that the plant was "completely out of control".
Any accident at the power station could have catastrophic consequences.
The assessment reflects the views of civilians in nearby Nikopol, which lies across the river and is still under Ukrainian control.
"Our forces don't shoot back because the 30km (19-mile) zone around the power station is sacred. You don't want to shoot there. But the Russians are terrorists. There's nothing sacred to them," the local businessman, who did not want to be named, told the BBC.
"It's meant to scare us," he continued, explaining that rockets have hit Nikopol every night since the middle of July.
A former employee of the plant, who is still in contact with colleagues but is now in Ukrainian-held territory, told the BBC that as well as firing rockets from the area around the plant, Russian forces had moved some military hardware into one of the main buildings.
The BBC can't verify the claim, but Enerhoatom has reported the same thing.
The UN's nuclear watchdog has warned several times about the difficult conditions for staff working at the power plant, and wants access to inspect the site.
The former employee, who worked at the plant for several weeks under Russian occupation before leaving, said the Russian soldiers in charge of the plant generally left the workers alone, but their presence caused psychological difficulties.
Many employees are unable to leave the occupied area because "they are afraid of losing their salaries, afraid to leave their relatives, or afraid of the Russians taking over their homes after they go".