Russia has accused the Ukrainian army of striking Europe's largest nuclear plant at Zaporizhzhia, accusing the government of Volodymyr Zelensky of acts of "nuclear terrorism".
Shelling hit a high-voltage power line at the major Ukrainian nuclear power station captured by Russia, prompting the plant's operators to disconnect a reactor despite no radioactive leak being detected.
Ukraine's state nuclear power company Energoatom blamed Russian shelling for the damage at the Zaporizhzhia power station.
"Ukrainian armed units carried out three artillery strikes on the territory of the Zaporizhzhia nuclear power plant and the city of Energodar," the Russian defence ministry said in a statement.
"We are urging international organisations to condemn the criminal actions of the Zelensky regime, which is carrying out acts of nuclear terrorism."
Russian troops took control of the plant in March after President Vladimir Putin sent troops to the pro-Western country on 24 February.
This week the International Atomic Energy Agency (IAEA) said that the situation at the nuclear power plant was "volatile".
Ukrainian President Volodymyr Zelensky said that Russia must take responsibility for what he described as an "act of terror".
"Today, the occupiers have created another extremely risky situation for all of Europe: they struck the Zaporizhzhia nuclear power plant twice. Any bombing of this site is a shameless crime, an act of terror," he said in his daily video address.
"Russia must take responsibility for the very fact of creating a threat to a nuclear plant."
Earlier, the Russian-installed administration of the occupied Ukrainian city of Enerhodar said Ukrainian shells struck the lines at the plant, in the country's southeast.
The Interfax news agency cited the city administration as saying fire had broken out on the plant's premises, and that power necessary for the safe functioning of reactors had been cut off.
The plant was captured by Russian forces in early March in the opening stage of the war.
Earlier this week, the United Nations nuclear watchdog appealed for access to the plant, which Washington says Russia is using as a battlefield shield.
Energoatom said the plant - located about 200km northwest of the Russian-held port of Mariupol - still worked and no radioactive discharges had been detected.
It was not the first time that military action has caused alarm at Zaporizhzhia, where the UN's International Atomic Energy Agency had at times reported losing connection with surveillance systems that keep track of nuclear material.
The administration said in a statement that fire had broken out and that power necessary for the safe functioning of reactors had been cut off. The plant continues to be run by its Ukrainian technicians.
Further east, both sides claimed small advances while Russian artillery bombarded towns and villages across a wide area in a now-familiar tactic.
Fighting on the ground appeared to be most intense around Pisky in Donetsk region, a fortified village held by Ukrainian troops and close to Donetsk city, which is in the hands of Russian-backed separatist forces.
The Russians also have the cities of Bakhmut and Avdiivka in their sights as they try to gain full control of the eastern Donbas area, Ukraine's industrial heartland.
In other developments, three grain ships left Ukrainian ports and the first inbound cargo vessel since the Russian invasion was due in Ukraine to load, marking further steps in the Kyiv government's efforts to resuscitate its economy after five months of war.
Russian President Vladmir Putin meanwhile was meeting Turkish President Tayyip Erdogan, who is cultivating a role as a mediator in the war, in the Russian city of Sochi.
"The international community cannot end the war in Ukraine by ignoring Russia," said Fahrettin Altun, a top aide to Erdogan.
Turkey helped negotiate the agreement that on Monday saw the first grain ship leave a Ukrainian port for foreign markets since the Russian invasion on 24 February.
Read the latest stories on Ukraine
Two grain ships set off from Chornomorsk and one from Odesa carrying a total of about 58,000 tonnes of corn, the Turkish defence ministry said.
The Turkish bulk carrier Osprey S, flying the flag of Liberia, was expected to arrive in Chornomorsk on Friday to load up with grain, the Odesa regional administration said.
Russia and Ukraine normally produce about one third of the world's wheat, and the United Nations had warned that the halt in grain shipments through the Russian-dominated Black Sea could lead to famine in other countries, particularly in Africa, Asia and the Middle East.
Disputed reports over control of Pisky
Since Russian troops poured over the border in February in what Putin termed a "special military operation", the conflict has settled into a war of attrition fought largely in the east and south of Ukraine.
Moscow is trying to gain control of the largely Russian-speaking Donbas, comprised of Luhansk and Donetsk provinces, where pro-Moscow separatists seized territory after the Kremlin annexed Crimea to the south in 2014.
Russia's TASS news agency cited separatist forces as saying they and Russian troops had taken full control of Pisky.
But Ukrainian presidential adviser Oleksiy Arestovych said: "There is very little evidence of any movement here. They (the Russians) made an attempt to advance but it was unsuccessful."
Ukraine has turned the village into a stronghold, seeing it as a buffer against Russian-backed forces holding Donetsk city about 10 km to the southeast.
Tass also said fighting was taking place in the city of Bakhmut, north of Donetsk and Russia's next main target.
"Russian forces may be advancing a few hundred metres a day. They are trying to encircle our forces," Mr Arestovych said.
Mr Arestovych also said Ukrainian forces had recaptured two villages near Izyum in Kharkiv region, which borders Russia, and were advancing on a third.
"This means Ukraine is on the offensive. It may not be a very big offensive. But it is an offensive nonetheless," he said.
Reuters could not verify either side's assertions about battlefield developments.
To the strategically-important south, where Ukraine has been planning a counteroffensive to win back swathes of occupied land, Russia has been building up forces, Kyiv says.
Russian troops may try to wrest momentum back from Kyiv by launching an offensive in the south after weeks of Ukraine using Western-supplied long-range weapons to hit Russian supply lines and ammunitions dumps.
The war has displaced millions, killed thousands of civilians and left cities, towns and villages in ruins. Ukraine and its Western allies have accused Russian forces of targeting civilians and war crimes, charges Russia rejects.
Mr Putin says he wants to ensure Russian security and protect Russian-speakers in Ukraine.
Kyiv accuses Moscow of an imperial-style war to retake a pro-Western neighbour that shook off Russian domination when the Soviet Union broke up in 1991.
Western countries and allies have piled financial restrictions on Russia since February.
Moscow retaliated with obstacles for Western businesses and their allies leaving Russia, and in some cases seized their assets.
In the latest move in the sanctions war, Russia on Friday banned investors from so-called unfriendly countries from selling shares in key energy projects and banks until the end of the year.
