The International Atomic Energy Agency on Wednesday said that external power at Zaporizhzhia nuclear plant in southern Ukraine was lost for the second time in five days.

The agency said that power was required for the functioning of vital safety systems at the nuclear power plant. It added that the plant was functioning on diesel generators kept as backup.

The Zaporizhzhia nuclear plant is Europe’s largest atomic power plant. It was captured by Russia in March, just weeks after President Vladimir Putin ordered his troops to invade Ukraine on February 24.

Fears regarding the safety of the nuclear plant have grown in recent times after both, Ukraine and Russia, have accused each other of shelling it.

Kyiv has repeatedly accused Russia of storing heavy weaponry inside the Zaporizhzhia nuclear plant complex and using it as a cover to launch attacks, knowing that Ukraine would not risk returning fire at the complex.

On Wednesday, International Atomic Energy Agency Director-General Rafael Grossi said that the loss of power at the nuclear plant is a deeply worrying development.

“It underlines the urgent need for a nuclear safety and security protection zone around the site,” he said.

Meanwhile, Ukraine’s state nuclear energy company Energoatom alleged that one of its convoys, which was carrying diesel fuel to the nuclear plant, was stopped by Russian forces, reported Reuters.

“Energoatom prepared and sent another batch of diesel fuel to the Zaporizhzhia nuclear plant,” the company said in a statement. “However, as of 10:00 am (0700GMT), the Russian side does not allow the company’s convoy of vehicles to pass.”