Energy supplies to Ukraine's nuclear plants have been affected as Russia has pounded its neighbour's power sector since the start of its 2022 invasion, prompting fears of a nuclear disaster.

The closed-door meeting started at 10:00 am (0900 GMT) at the Vienna headquarters of the International Atomic Energy Agency (IAEA), AFP journalists said.

"We expect strong signals of support (and an) evaluation of the situation," Ukrainian ambassador Yurii Vitrenko told reporters before the meeting started.

He added it was "high time" for the IAEA board meeting "to take up this situation."

A weeks-long IAEA expert mission to Ukrainian substations and power plants is underway and expected to wrap up next month, Vitrenko said.

Following a series of attacks, 13 countries led by the Netherlands in a letter asked the IAEA board to "meet with regard to the latest developments in Ukraine and their nuclear safety implications".

"Our delegations share a growing concern about the severity and urgency of nuclear safety risks posed by this situation," the IAEA members said in the letter dated January 21 seen by AFP.

"The latest attacks have caused significant additional damage to energy infrastructure in Ukraine which is deemed essential for the safe operation of nuclear power plants," the letter added.

Last week, Ukraine's Chernobyl nuclear power plant temporarily lost all off-site power.

Europe's biggest nuclear power plant, Zaporizhzhia, occupied by Russian forces since March 2022, has also been repeatedly affected by fighting.

Earlier this month, Russia and Ukraine agreed to a localised ceasefire to allow repairs on the last remaining backup power line supplying Zaporizhzhia.

The line was damaged and disconnected as a result of military activity in early January.

The Zaporizhzhia plant's six reactors have been shut down since the occupation. But the site still needs electricity to maintain its cooling and security systems.

Moscow and Kyiv have repeatedly accused each other of risking a nuclear catastrophe by attacking the site.