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The UN atomic watchdog chief on Tuesday described radiation levels at the nuclear disaster site as "abnormal", saying the area's brief Russian occupation had been "very, very dangerous".

"The radiation level, I would say, is abnormal," said director during a visit on the anniversary of the world's worst nuclear disaster.

"There have been some moments when the levels have gone up because of the movement of the heavy equipment that Russian forces were bringing here, and when they left," he said.

"We are following that day by day."

Speaking as he arrived at the sarcophagus that covers the nuclear reactor's radioactive remains, he said the takeover by Russian forces had been "absolutely abnormal and very, very dangerous".

Russian troops took over the site on February 24, the first day of Russia's invasion of Ukraine, taking Ukrainian soldiers prisoner and detaining civilian staff at the site.

The occupation lasted until the end of March and raised global fears of nuclear leaks.

Ukrainian officials have said that Russian soldiers may have been exposed to radiation after digging fortifications in "many places" at the site and stirring up clouds of dust with their armoured vehicles.

On April 26, 1986, an uncontrolled nuclear chain reaction destroyed the reactor in an accident that was initially covered up by the Soviet authorities.

Many hundreds died though the exact figure remains disputed.

Eventually, 350,000 people were evacuated from a 30-kilometre (19-mile) radius around the plant, an exclusion zone that remains uninhabited, apart from some elderly residents who returned despite an official ban.

The Chernobyl power station's three other reactors were successively closed, with the latest shutting off in 2000.