The Zaporizhzhia nuclear power plant should have enough water to cool its reactors for "some months" from a pond located above the reservoir of a nearby dam that has broken, the U.N. atomic watchdog said on Tuesday, calling for the pond to be spared.

The major Soviet-era dam near the Russian-held nuclear plant in southern Ukraine was breached on Tuesday, unleashing floodwaters across the war zone in what both Ukraine and Russia said was an intentional attack by the other's forces.

The dam's reservoir provided water used for the essential cooling of the six reactors at Europe's biggest nuclear power plant as well as of spent fuel and emergency diesel generators that have had to be used repeatedly when external power fails.

"There are a number of alternative sources of water. A main one is the large cooling pond next to the site that by design is kept above the height of the reservoir," International Atomic Energy Agency chief Rafael Grossi said in a statement issued in response to the breach of the Kakhovka dam.

Water from the pond should provide enough cooling water for "some months," Grossi said, adding that his agency would confirm that "very shortly."

"It is therefore vital that this cooling pond remains intact. Nothing must be done to potentially undermine its integrity. I call on all sides to ensure nothing is done to undermine that," Grossi said.

While he had already been due to visit the Zaporizhzhia plant next week, that visit had now become essential and would go ahead, he added.