This is Grossi's first visit to Ukraine after Russia seized several nuclear facilities, including Chernobyl, during its more than month-long invasion.

"The aim of the director general's visit is to initiate prompt safety and security support to Ukraine's nuclear facilities," the International Atomic Energy Agency (IAEA) said in a statement.

Grossi is holding talks with senior government officials and will travel to one of Ukraine's nuclear power plants during the week's visit, the agency said, without specifying which one.

The visit includes the shipment of "vital safety and security supplies" including monitoring and emergency equipment.

"The military conflict is putting Ukraine's nuclear power plants and other facilities with radioactive material in unprecedented danger," Grossi said.

"Ukraine has requested our assistance for safety and security. We will now start delivering it," he said.

Since the start of the Russian military offensive, the IAEA chief has repeatedly warned of the dangers of the conflict -- the first to take place in a country with a vast nuclear estate comprising 15 reactors, in addition to Chernobyl, the site of the world's worst nuclear disaster.