Those four power plants, located near towns and cities in western and southern Ukraine, contain 15 operating nuclear reactors. The concern is that a nuclear disaster could occur if during the conflict they were accidentally damaged, left unmaintained or cut off from the power grid needed to cool the reactors.

Ukraine reported that its country’s nuclear power reactors are still operating “safely and securely,” the International Atomic Energy Agency said Friday morning.

But the organization, which is dedicated to the peaceful use of nuclear energy, said it remained deeply concerned.

IAEA Director General Rafael Mariano Grossi reiterated his “appeal for maximum restraint to avoid any action that could jeopardize plant safety,” the group said.