"The situation was absolutely abnormal and very, very dangerous," Rafael Grossi told reporters as he arrived at the sarcophagus that covers the nuclear reactor's radioactive remains.

Grossi, head of the International Atomic Energy Agency, was visiting the site on the 36th anniversary of the world's worst nuclear disaster.

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.

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

Grossi said radiation levels were now "normal".

But he added that "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".

Ukrainian officials have said 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, 3,50,000 people were evacuated from a 30-kilometre 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.