Ukraine and Russia have blamed each other after a fire broke out at the giant Zaporizhzhia nuclear power plant on Sunday.
Ukraine's President Volodymyr Zelensky said Russian forces had started a fire at the plant, which has been occupied by Moscow's forces for more than two years. Zaporizhzhia's Kremlin-installed governor said Ukrainian shelling caused the blaze.
The UN's nuclear watchdog said it saw "strong dark smoke" coming from the facility - but said there was "no impact reported" for nuclear safety.
The development comes as Ukrainian troops have advanced up to 30km inside Russia, in the deepest and most significant incursion since Moscow began its full-scale invasion in February 2022.
On Sunday, Yevgeny Balitsky, the Kremlin-installed governor of Zaporizhzhia, said a fire had broken out at the cooling towers of the power plant.
He blamed Ukrainian shelling, but asked for "calm", adding that there had been no radiation spike around the plant.
Mr Zelensky also said there was no detected radiation spike or danger of a nuclear leak - but accused Russia of purposefully starting the fire in an attempt to "blackmail" Kyiv.
In the early hours of Monday, Vladimir Rogov, another Kremlin-installed official, said the fire had been "completely extinguished" in a Telegram post.
The nuclear power plant has been under the control of Russian troops and officials since 2022. It has not produced power in more than two years and all six reactors have been in cold shutdown since April.
In a statement posted on X, UN nuclear watchdog the International Atomic Energy Agency (IAEA) said its experts had witnessed "strong dark smoke" coming from the plant following "multiple explosions".
It said the plant had reported "an alleged drone attack" on one of the cooling towers at the site.
"No impact has been reported for nuclear safety," the IAEA added.
In a later statement, the IAEA said it had requested "immediate access to the cooling tower to assess the damage".
The fire comes just a day after President Zelensky acknowledged for the first time that his military is conducting a cross-border offensive inside Russia's western Kursk region.
In his nightly video address on Saturday, Mr Zelensky said Ukraine's military was pushing the war onto "the aggressor's territory".
A senior Ukrainian official told the AFP news agency that thousands of troops were engaged in the operation, far more than initially reported by Russian border guards.
Kyiv launched its surprise attack on Tuesday, rapidly advancing more than 30km (19 miles) inside Russia - the deepest raid since Moscow began its full-scale invasion of Ukraine in February 2022.
Russia has so far struggled to halt the Ukrainian advance, with more than 76,000 people evacuated from the Kursk region, where a state of emergency has been declared by local authorities.
Emergency trains from Kursk to Moscow have also been set up by Russia's rail operator for those looking to flee.
The fighting appeared to be ongoing over the weekend, with Kursk Governor Aleksei Smirnov saying early on Sunday that there were injured people in a "treacherous" Ukrainian attack.
Earlier this week, Russian President Vladimir Putin said the offensive was a "major provocation".
