Grossi has repeatedly warned of the dangers of the conflict -- the first in a country with a vast nuclear estate.

Russian forces seized control of the defunct Chernobyl nuclear power plant site -- where radioactive waste is still stored -- on February 24, the first day of the invasion.

They also captured Europe's largest nuclear plant at Zaporizhzhia on March 4, sparking alarm when shelling caused a fire at a training facility.

"I'm at South Ukraine Nuclear Power Plant to meet Ukrainian government officials and staff, and start IAEA technical assistance for safety and security of country's nuclear facilities," the International Atomic Energy Agency head wrote on Twitter on Wednesday.

"Vital to be on the ground to provide effective support to in these extremely difficult times."

Grossi also thanked the staff of the facility, also called the Yuzhnoukrainsk nuclear plant, for "their endurance and resilience".

"I want to say that we are here with you, that we are ready to support you in whatever way and form we can," he said in a video message also posted on Twitter.

The director of the Ukrainian energy firm Energoatom said Wednesday that Russian actions were jeopardising safety.

"Due to these actions by the invaders, IAEA norms are being violated on a daily basis" at Yuzhnoukrainsk and Chernobyl and the situation is "getting worse", Energoatom director Petro Kotin said in the statement.

"As long as these installations are under the control of Russian invaders the entire world is in danger," he added.

Meanwhile, Deputy Prime Minister Iryna Vereshchuk said that "the Russian occupiers have created an enormous ammunition depot in the exclusion zone around Chernobyl."

She warned it could explode at any moment and cause a "colossal environmental catastrophe".

She called on the UN Security Council to send a special UN mission to remove military forces for the 30-kilometre (20-mile) zone around the nuclear power plant.

Grossi began his Ukraine visit on Tuesday to meet government officials, as well as send experts and equipment to the country "to help prevent the danger of a nuclear accident".

The IAEA has not said how long its chief will stay in Ukraine.

Ukraine has 15 reactors at four active nuclear power plants, as well as stores of nuclear waste, including at Chernobyl -- the site of the world's worst nuclear disaster in 1986.

Grossi met Ukrainian and Russian foreign ministers in Turkey earlier this month to discuss nuclear safety but no agreement has been reached yet.

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