A fire that threatened potential disaster at Ukraine's largest nuclear power plant following an attack by Russian troops has been extinguished, authorities said. According to Ukraine's state emergency services (SES), it started after Russian military units fired on a training building outside the main reactor complex of the Zaporizhzhia nuclear power plant.

Leading nuclear authorities were concerned — but not panicked — about the damage to the power station. The assault triggered phone calls between Ukrainian president Volodymyr Zelenskyy and U.S. president Joe Biden and other world leaders. The U.S. department of energy activated its nuclear incident response team as a precaution.

Nuclear plant spokesman Andriy Tuz told Ukrainian television that shells were falling directly on the facility and had set fire to one of its six reactors. That reactor is under renovation and not operating, but there is nuclear fuel inside, he said.

The assault renewed fears that the invasion could damage one of Ukraine’s 15 nuclear reactors and set off another emergency like the 1986 Chernobyl accident, the world’s worst nuclear disaster, which happened about 110 kilometers (65 miles) north of the capital.

U.S. energy secretary Jennifer Granholm tweeted that the Zaporizhzhia plant’s reactors were protected by robust containment structures and were being safely shut down.

The international atomic energy agency said the fire had not affected essential equipment and that Ukraine’s nuclear regulator reported no change in radiation levels. The American Nuclear Society concurred, saying that the latest radiation levels remained within natural background levels.

Jon Wolfsthal, who served during the Obama administration as the senior director for arms control and nonproliferation at the National Security Council, said the plant’s reactors have thick concrete containment domes that should protect them from tank and artillery fire.