Russian forces pressed their attack on a crucial energy-producing Ukrainian city by shelling Europe’s largest nuclear plant early Friday, sparking a fire and raising fears that radiation could leak from the damaged power station, Associated Press reported.

The assault on the eastern city of Enerhodar and its Zaporizhzhia nuclear plant came as the invasion entered its second week with Russian forces gaining ground in their bid to cut off the country from the sea. Elsewhere, another round of talks between the two sides yielded a tentative agreement to set up safe corridors inside Ukraine to evacuate citizens and deliver humanitarian aid.

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.

Firefighters cannot get near the flames because they are being shot at, he said, and Ukrainian Foreign Minister Dmytro Kuleba tweeted a plea to the Russians to stop the assault and allow fire teams inside.

“We demand that they stop the heavy weapons fire,” Tuz said in a video statement. “There is a real threat of nuclear danger in the biggest atomic energy station in Europe.”

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

The American Nuclear Society condemned the attack but said the latest radiation levels remained within natural background levels.

“The real threat to Ukrainian lives continues to be the violent invasion and bombing of their country,” the group said in a statement from President Steven Nesbit and Executive Director and CEO Craig Piercy.

The plant’s reactor is a different type than the one used at Chernobyl, and there should be little risk if the containment vessel is not damaged and outside power can be restored, said Jon B. Wolfsthal, a former senior director for arms control and nonproliferation at the National Security Council and former special adviser to then-Vice President Joe Biden.

“Everyone needs to take a step back and not jump to conclusions,” Wolfsthal, now a senior adviser at Global Zero, said on Twitter.

The International Atomic Energy Agency said it was in contact with authorities in Ukraine. The agency’s director general, Mariano Grossi, urged military forces to refrain from violence near the plant.