"The news from our team yesterday and this morning is extremely disturbing," Rafael Grossi, head of the International Atomic Energy Agency, said on Sunday.
"Explosions occurred at the site of this major nuclear power plant, which is completely unacceptable.
"Whoever is behind this, it must stop immediately. As I have said many times before, you're playing with fire."
There were more than a dozen blasts over Saturday night, some of which a team of experts from the agency were witness to, the IAEA said.
Mr Grossi told French broadcaster BFMTV later on Sunday it was clear that the strikes on the plant were no accident.
"The people who are doing this know where they are hitting. It is absolutely deliberate, targeted," he said.
Ukraine capital Kyiv hit by kamikaze drones - in pictures
Ukraine meanwhile dismissed Moscow's accusations that its soldiers had killed Russian troops as they were surrendering, in what Moscow has described as a "war crime".
Dmytro Lubinets, Ukraine's commissioner for human rights said on Sunday that extracts from the video footage in question showed that Russian troops used a "staged surrender" to open fire on Ukrainian soldiers.
"In this case, persons among the Russian servicemen cannot be considered prisoners of war, but are those who are fighting and committing treachery," Mr Lubinets said.
"Returning fire is not a war crime. On the contrary, those who want to use the protection of international law to kill must be punished."
Ukraine war latest - in pictures
Video circulated on Russian social media last week purported to show the bodies of Russian servicemen killed after surrendering to Ukrainian troops.
The videos have not been independently verified.
A UN spokesman told AFP on Friday that the world body was "aware of the videos" and "looking into them".
Meanwhile In the southern city of Kherson, which Ukrainian troops recently recaptured, residents were facing a new challenge after eight months of occupation by Moscow's troops — Russian artillery attacks.
After Russian shells pounded the industrial area next to their home, setting fire to an oil depot there, Yuri Mosolov and his wife decided it was time to leave.
"After yesterday's shelling, my wife said: 'Let's not take too many risks and go'," Mr Mosolov said.
A carefully planned campaign by Kyiv against logistics networks, bridges and pontoon crossings battered Russian supply lines and forced their troops to abandon the city and retreat to the Dnipro's eastern bank.
Now the armies are increasingly engaging in heavy artillery exchanges across the river.
"Artillery duels are still going on. The combat continues," said Dmytro Pletenchuk, the Ukrainian military's spokesman in the area. "Kherson is now on the frontline."
Antonov AN-225 destroyed by Russia in Ukraine - in pictures
Elsewhere near Kherson, Russian strikes hit near a humanitarian distribution area in the village of Bilozerka, sending residents fleeing on Saturday.
Ukraine's President Volodymyr Zelenskyy, in his daily address, said there had been nearly 400 Russian attacks in the east of the country alone on Sunday.
Mr Zelenskyy said the toughest battles were in the eastern Donetsk region, one of those that Russian now claims as its own. Fighting was also continuing in neighbouring Luhansk.
Teams were working around the clock to repair damage to the energy infrastructure from Russia's artillery bombardments, but "stabilisation blackouts" were nevertheless necessary in 15 regions, including Kyiv, he added.
And Mr Zelenskyy again set out Kyiv's terms for peace, which include food and energy security, the release of all prisoners and deportees, and the withdrawal of Russian troops from all Ukrainian territory.