"The situation was absolutely abnormal and very, very dangerous," International Atomic Energy Agency director general Rafael Grossi told reporters.

Grossi spoke as he arrived at the sarcophagus that covers the radioactive remains of reactor number four of the former nuclear power plant.

Russian troops took over the site of the world's worst nuclear disaster on February 24, the first day of its invasion of Ukraine, staying there for several weeks and raising 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 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 power station's three other reactors were successively closed, with the latest shutting off in 2000.