The UN's nuclear watchdog has called for an immediate end to any military action near Ukraine's Zaporizhzhia nuclear plant, warning of a "very real risk of a nuclear disaster".
IAEA chief Rafael Grossi said he was "extremely concerned" by reports of shelling at Europe's largest nuclear power plant.
It comes as Ukraine said parts of the facility were "seriously damaged" by Russian military strikes.
Russia seized the plant in March.
Russia has kept on its Ukrainian employees, but Kyiv accuses Russian forces of firing rockets at civilian areas from the site, employing "terror tactics".
Friday's strikes underline "the very real risk of a nuclear disaster that could threaten public health and the environment in Ukraine and beyond", Mr Grossi, director general of the International Atomic Energy Agency, said, in a statement.
"Any military firepower directed at, or from, the facility would amount to playing with fire, with potentially catastrophic consequences," he added.
Ukrainian staff must be able to carry out their important duties "without threats or pressure", he said, adding that the IAEA should be allowed to provide technical support.
"For the sake of protecting people in Ukraine and elsewhere from a potential nuclear accident, we must all set aside our differences and act, now. The IAEA is ready," said Mr Grossi, days after stating the plant was "completely out of control".
The operator of the Zaporizhzhia plant said the Russian missile strikes had forced the closure of one "power unit", adding that there was a risk of radioactive leaks.
The strikes "caused a serious risk for the safe operation of the plant", operator Enerhoatom wrote on Telegram.
Moscow said Ukraine carried out the attack.
The BBC was unable to verify the reported damage at the nuclear plant.
However, the EU has hit out at Moscow over the latest shelling - with the bloc's top diplomat, Josep Borrell, saying it "condemns Russia's military activities" around the plant.
"This is a serious and irresponsible breach of nuclear safety rules and another example of Russia's disregard for international norms," he said, and called for the IAEA to be granted access to the plant.
Russian forces hold the plant and surrounding areas, close to Ukrainian-held territory. It consists of six pressurised water reactors and stores radioactive waste.
Civilians in nearby Nikopol, which lies across the river and is still under Ukrainian control, told the BBC that the Russians were firing rockets from the area around the plant and moving military hardware into the compound.
Ukraine's President Zelensky said, on Friday, that "any bombing of this site is a shameless crime, an act of terror".
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 UK defence ministry says Russia is 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.