Ukraine must "prepare for all scenarios" linked to its Russian-occupied nuclear plant, the interior minister said during a drill for emergency workers in the nearby city of Zaporizhzhia.
Moscow and Kyiv have accused each other of shelling the Zaporizhzhia plant, the biggest atomic power station in Europe, which has been under Russian control since March.
The tensions around the facility have sparked fears of another nuclear disaster in Europe like the one in Chernobyl in then Soviet Ukraine in 1986.
"Nobody could have predicted that Russian troops would be firing on nuclear reactors with tanks. It is incredible," Interior Minister Denys Monastyrsky said in the Ukrainian-held city of Zaporizhzhia, 50 kilometers (31 miles) from the plant.
After watching a nuclear accident drill on Wednesday, he said Ukraine should be prepared since "as long as the Zaporizhzhia nuclear plant is controlled by Russia, there are major risks."
Dozens of Ukrainian emergency workers wearing gas masks and hazmat suits took part in the drill in which they practiced evacuating an injured person and washing down contaminated vehicles.
The head of Ukraine's state nuclear agency Energoatom, Petro Kotin, said around 500 Russian soldiers and 50 armored vehicles were at the plant.
Russia said it does not have any heavy weapons or troops there except for guard units.
Ukraine has accused Russia of firing on the plant in order to accuse Ukraine of doing it, while Russia has accused Kyiv of shelling the facility.
'Deadlocked'
Ukrainian forces said on Thursday they had beaten back a Russian attack in the southern region of Kherson, while the death toll from the Russian shelling of the city of Kharkiv in Ukraine's northeast climbed as the nearly six-month war grinds on without let-up.
They will discuss ways to find a political solution to the war and address the threat to global food supplies and risk of a disaster at Europe's largest nuclear power plant, which has been taken over by Russian forces.
The war has forced millions to flee, killed thousands and deepened a geopolitical rift between the West and Russia, which says the aim of its operation is to demilitarize its neighbor and protect Russian-speaking communities.
"Russian forces have achieved only minimal advances, and in some cases we have advanced, since last month," Ukrainian presidential adviser Oleksiy Arestovych said in a video.
"What we are seeing is a 'strategic deadlock.'"
Russia's Defense Ministry, meanwhile, accused Ukraine on Thursday of planning a "provocation" at the Zaporizhzhia nuclear power plant on Aug. 19 during a visit by U.N. Secretary-General Antonio Guterres to Ukraine, Russian state-owned news agency RIA reported.
The ministry provided no evidence to back up its assertion.
In a statement, it said there are no Russian heavy weapons at the Russian-controlled nuclear reactor complex, or in nearby districts.
The plant has come under fire repeatedly in recent weeks, with both Ukraine and Russia blaming each other for the shelling. Ukraine has said that Russia has deployed artillery in and around the plant.
Kharkiv shelling
Two civilians were killed and 18 wounded in a predawn rocket attack on the eastern Ukrainian city of Kharkiv on Thursday, regional governor Oleh Synehubov said.
The strike followed a Russian attack on Kharkiv on Wednesday in which Synehubov said seven civilians were killed and 17 wounded.
"Last night was one of the most tragic of the entire war in the Kharkiv region," he wrote on the Telegram messaging app.
Zelenskyy described Wednesday's attack on Kharkiv, Ukraine's second-largest city, as a "devious and cynical strike on civilians with no justification."
"We cannot forgive. We will avenge it," he said.
Pavlo Kyrylenko, the governor of the eastern region of Donetsk, said on Telegram that three civilians had been killed in Russian attacks in the region in the past 24 hours.
Black Sea fleet chief replaced
The United States, Albania, France, Ireland, Norway and Britain have asked the U.N. Security Council to meet on Aug. 24 to discuss the impact of the war in Ukraine, diplomats said, marking six months since Russia's invasion.
A series of blasts at military bases and ammunition depots in the past week in Crimea, the Black Sea peninsula annexed by Russia in 2014, has suggested a shift in the conflict, with Ukraine apparently capable of striking deeper into Russian-occupied territory.
Russia blamed saboteurs for the attacks, while Ukraine has not officially taken responsibility but has hinted at it.
Ukrainian military intelligence said in a statement that after the recent explosions in Crimea, Russian forces had urgently moved some of their planes and helicopters deeper into the peninsula and to airfields in Russia. Reuters could not independently verify the information.
On Wednesday, Russia's RIA news agency cited sources as saying the commander of its Black Sea Fleet, Igor Osipov, had been replaced with a new chief, Viktor Sokolov.
If confirmed, it would mark one of the most prominent sackings of a military official in a war in which Russia has suffered heavy losses of men and equipment.
The Black Sea Fleet, which has a revered history, has suffered several humiliations since President Vladimir Putin launched the invasion of Ukraine – which Moscow calls a "special military operation" – on Feb. 24.
In April, Ukraine struck Russia's flagship the Moskva, a huge cruiser, with Neptune missiles. It became the biggest warship to be sunk in combat for 40 years.