Rafael Grossi, Director General of the IAEA, with a map of the Ukrainian nuclear power plant. /Joe Klamar/AFP
The nuclear plant at Zaporizhzhia has been seized by Russian forces, according to local reports.
Overnight shelling of the area resulted in a fire that burned for several hours, launching a wave of protests worldwide over fears that Europe's largest nuclear facility might be in danger of damage or even meltdown.
A spokesman at the plant put out an emergency appeal to the Russians to stop their bombardment and allow crews to tackle the fires.
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World leaders were quick to protest. U.S. President Joe Biden and Canada's Prime Minister Justin Trudeau urged Moscow to stop its military action around the site.
UK Prime Minister Boris Johnson said the "reckless" attack could directly threaten the safety of all of Europe.
In Ukraine, President Zelenskyy rebuffed the Russian version that the fire was the work of Ukrainian saboteurs. "If there is an explosion," he said, "it is the end of everything."
He invoked the memory of Chernobyl, the Ukrainian nuclear reactor which suffered a massive meltdown in 1986, causing widespread radioactive pollution. The incident is considered the world's worst nuclear disaster.
The International Atomic Energy Agency (IAEA) confirmed that a missile had hit one of the buildings near the reactors.
But IAEA director-general Rafael Mariano Grossi said that while the situation remains challenging, the safety systems of the six reactors at the plant were not affected, and there had been no release of radioactive material.
Fighting remains fierce in the southeast of Ukraine, where the reactor is located. Nuclear experts are growing increasingly concerned about the proximity of heavy weapons to such installations.
"When you have active nuclear power plants, it's just a recipe for disaster," said nuclear expert Maria Rost Rublee. "All it takes is one missile."
At a NATO meeting on Friday, member states rejected Ukraine's demand for no-fly zones saying they were increasing support, but direct involvement would lead to a broader, even more, brutal European war.