He has already issued warnings over the situation at both the Russian-occupied Zaporizhzhia nuclear power plant and the Kursk power plant, near to where Kyiv has mounted its incursion into Russia.

Grossi met the head of Russia's Rosatom nuclear agency Alexei Likhachev in Kaliningrad, after visiting the Zaporizhzhia plant in southeast Ukraine this week and the Kursk plant the week before.

He met with Ukrainian leader Volodymyr Zelensky on Tuesday.

"The times remain very difficult," Grossi told Likhachev, Russian news agencies reported.

The situation at the Kursk power plant -- some 50 kilometres (30 miles) from Kyiv's forces -- was "worrying", he said.

But both officials said the plant's functioning was stable, and Grossi again urged both sides to refrain from attacking power plants.

"I said this in Zaporizhzhia, I said this in Kyiv and now I say this in Kaliningrad: power plants can never be legitimate targets in an armed conflict," he said.

Russian President Vladimir Putin has accused Ukraine of trying to strike the Kursk plant, without providing evidence.

Likhachev said Russia "expects an adequate response" from the International Atomic Energy Agency.

Ukraine's incursion into Kursk is now a month long, with Kyiv laying claim to dozens of Russian border settlements.

Putin said this week that Moscow has "gradually" started to push out Kyiv's forces from Kursk.

The Zaporizhzhia plant fell to Moscow in the first days of its offensive in 2022.