Fire breaks out at a nuclear power complex during a Russian assault, Ukraine says.

The Ukrainian foreign minister, Dmytro Kuleba, had warned that the fire posed a significant risk.

“If it blows up, it will be 10 times larger than Chernobyl!” he said on Twitter, referring to the disaster at that nuclear site in 1986. “Russians must immediately cease fire and allow firefighters to establish a security zone,” he said.

Prime Minister Boris Johnson of Britain said he would seek an emergency meeting of the United Nations Security Council about the blaze at the complex, according to his office.

Rafael Mariano Grossi, the director general for the International Atomic Energy Agency, said on Twitter that he had spoken with Ukrainian officials about the situation at the plant. He appealed for a halt to the fighting and warned of “severe danger” if the reactors were hit.

The American Nuclear Society condemned the Russian attack on the reactor complex.

Earlier, Mr. Grossi had said that “a large number of Russian tanks and infantry” had entered Enerhodar, a town next to the plant, and that infantry troops were “moving directly” toward the reactor site.

The mayor, Dmitry Orlov, told a local radio station that fierce fighting between Russian and Ukrainian soldiers had been raging on the approaches to the plant, according to the station’s Twitter account. The mayor called for an immediate cease-fire.

The Zaporizhzhia nuclear complex, on the Dnieper River roughly a hundred miles north of Crimea, is the largest in Europe. According to the International Atomic Energy Agency, its six reactors produce a total of 6,000 megawatts of electric power.

In comparison, the Chernobyl plant in northern Ukraine produced 3,800 megawatts — about a third less. (A megawatt, one million watts, is enough power to light 10,000 hundred-watt bulbs.) The four reactors of the Chernobyl complex were shut down after one suffered a catastrophic fire and meltdown in 1986.