Former world heavyweight boxing champion Volodymyr Klitsyko warned that fighting at the Zaporizhia nuclear power plant in Ukraine could lead to a much bigger disaster than those that took place in Fukushima and Chernobyl, dpa reported on Sunday, Agerpres reported.

Volodymyr Klitsiko, the younger brother of Kyiv Mayor Vitaliy Klitsiko and a former professional boxer, said it was unclear how Ukrainian personnel were holding up at the plant seized by Russian forces.

“The world must understand that if (the Zaporizhzhya NPP) explodes, it will have the consequences (of the Fukushima and Chernobyl disasters) several times over,” Volodymyr Klitsyko told the British Times radio station.

According to Volodymyr Klytsiko, the station and the nuclear experts who work there are “hostages” of Moscow.

Zaporizhzhia NPP, Europe’s largest nuclear power plant with six reactors, has been occupied by Russian troops since the beginning of March. It is of strategic importance for providing Ukraine with electricity.

United Nations Secretary-General Antonio Guterres, who was in that country on Friday, appealed to Russia not to disconnect the Zaporizhzhia NPP from the Ukrainian power grid after the Ukrainian operator Energoatom caused Russia to do so.

Russian troops occupied the area around the Zaporizhzhia power plant in March, shortly after Russia sent tens of thousands of troops into Ukraine. The plant, which is still staffed by Ukrainian personnel, remains close to the front line and has come under fire several times in recent weeks, raising fears of a nuclear disaster. Ukraine and Russia have blamed each other for explosions in and around a nuclear reactor complex in southern Ukraine.

Russia has rejected international calls to withdraw its troops from the plant, but has said it will allow a panel of experts from the International Atomic Energy Agency (IAEA) to access the plant.

Ukrainian President Volodymyr Zelenskyi said on Thursday that the IAEA mission should only be present in territories not occupied by Russia, demanding that Russian troops first be withdrawn from the site of the nuclear power plant before the IAEA mission can arrive. Kyiv insists on the creation of a demilitarized zone around the NPP, and Moscow considers this request unacceptable.