Moscow meanwhile said it was gaining ground in east Ukraine, while Russian officials accused Kyiv of attacking and sinking a cargo ship moored at a port near the annexed Crimean peninsula.

"The enemy tried to strike the nuclear power plant at night. The International Atomic Energy Agency (IAEA) has been informed," Putin said during a televised government meeting on Thursday.

Putin did not present any evidence for his claims or provide further details on the alleged attack.

The IAEA released a statement saying it had been told by Moscow drone fragments were found roughly 100 metres from the Kursk plant's spent fuel nuclear storage facility.

The nuclear watchdog said its chief would visit the facility next week, with Russia having repeatedly sounded the alarm over a possible hit since Ukrainian troops and tanks stormed into its western Kursk region on August 6.

That offensive is now into its third week, with Kyiv laying claim to dozens of Russian border settlements and Russia scrambling to fight off the most serious attack by a foreign army on its territory since World War II.

There were no previous reports of the attempted strike on the facility in Russian media.

Kursk regional governor Alexei Smirnov told Putin the facility was working as usual.

There was no immediate reaction from officials in Kyiv to Putin's claim.

- 'Maximum restraint' -

Ukraine and Russia have traded accusations of threatening nuclear safety throughout the two-and-a-half-year conflict.

Russian troops seized the abandoned Chernobyl power plant in northern Ukraine and the Zaporizhzhia nuclear power plant -- Europe's largest -- in the first days of its full-scale military offensive.

It still controls the Zaporizhzhia plant, and has been accused of "nuclear blackmail" by Ukrainian President Volodymyr Zelensky.

Moscow, in turn, claims Ukrainian forces have tried to strike the plant with drones on multiple occasions.

Earlier this month a fire broke out in one of the Zaporizhzhia plant's cooling towers.

Russia said it was the result of a Ukrainian attack, while Kyiv said Russia had purposefully started the blaze.

After Ukraine launched its armed incursion into the Kursk region, the IAEA urged both Russia and Ukraine to exercise "maximum restraint" to "avoid a nuclear accident with the potential for serious radiological consequences".

Separately, a Ukrainian aerial attack sank a cargo ferry in southern Russia on Thursday, Russian officials said.

The ferry was carrying fuel tanks and was docked at the port of Kavkaz, which sits in the Kerch Strait that separates Russia from the annexed Crimean Peninsula.

Officials in Kyiv posted cryptic comments after the attack.

"Beautiful," Daria Zarivna, a communications adviser to Ukrainian President Volodymyr Zelensky, said in a post on Telegram, attaching a photo of a large fire at the port.

- 'Strengthen defence' -

Zelensky on Thursday visited his commander-in-chief in the Sumy border region, from where Ukraine is pouring troops and military hardware into Russia.

Kyiv's troops are still facing grinding setbacks in the eastern Donetsk region, where they have been under pressure for months.

On Thursday, Russia claimed to have captured another small village there.

In Russia, Putin has been accused of publicly downplaying the seriousness of the Ukrainian incursion.

Kursk governor Smirnov said on Thursday that 133,000 people have fled or been evacuated from border districts since Ukraine launched the attack.

Concrete air-raid shelters were being installed in cities across the region on Thursday, including Kurchatov, next to the Kursk Nuclear Power Plant.

And more than 110 Russian schools located close to the border will teach classes remotely when the academic year starts in September, the education minister said on Thursday.

Russia's FSB security service said on Thursday that it had launched a criminal case against several journalists, including CNN correspondent Nick Paton Walsh, for having "illegally" crossed the border after they reported from the Kursk region.