The head of the International Atomic Energy Agency says he will lead a team of experts to the Zaporizhzhia nuclear power plant in southeastern Ukraine in a few days.

IAEA Director General Rafael Mariano Grossi made the remarks in an interview with French media on Thursday.

Russian troops have taken control of the nuclear power plant, the largest plant in Europe. Russia is believed to be reinforcing troops in the compound.

The plant has been under constant shelling. Ukraine's state energy company Energoatom said on Thursday that power lines to the plant were severed due to nearby fires.

Ukrainian President Volodymyr Zelenskyy said the IAEA and other world bodies need to act much faster because every minute that the Russia troops remain at the power station poses a risk of global nuclear catastrophe.

Meanwhile, Russian President Vladimir Putin signed a decree on Thursday to raise the number of military personnel by 137,000. This would bring the total to roughly 1.15 million troops. The order will go into effect on January 1, next year.

The US Institute for the Study of War said on Thursday that the decree is unlikely to generate significant combat power in the near future and indicates that Putin is unlikely to order a mass mobilization soon.

Britain's Defence Ministry has refuted a comment by Russia's Defence Minister Sergei Shoigu that Russia was deliberately slowing the pace of its military campaign to minimize civilian casualties.

The British ministry said the Russian offensive has stalled because of its poor military performance and fierce Ukrainian resistance. It also said it is highly likely that at least six Russian generals have been fired for not advancing quickly enough.