British Prime Minister Boris Johnson said Russian President Vladimir Putin’s “reckless actions…could now directly threaten the safety of all of Europe.” Canadian Prime Minister Justin Trudeau said the attacks were “unacceptable” and “must cease immediately,” and Ukrainian President Volodymyr Zelenskyy accused Russia of unprecedented “nuclear terrorism.”

The attack came early Friday morning. Initial reports of the fire were unclear as to its location, but Ukraine’s emergency services said at 6:20 a.m. local time that it had been in the plant’s “training building” and was extinguished. They previously reported that the Russians had blocked them from responding to the blaze, which raged for around four hours.

At the time of publication, fighting had reportedly stopped around the plant, and Russian troops had taken control, though all operational staff are still Ukrainian. Most of Zaporizhzhia’s six reactors are now being cooled down, and one remains operational at 60% capacity.

“The Zaporizhzhia NPP site has been seized by the military forces of the Russian Federation,” Ukraine’s nuclear regulator said in a statement. “The personnel continue working at their workplaces, operational personnel monitor the state of power units and ensure their operation in accordance with the requirements of the process procedures for safe operation. Walk downs are also carried out to identify any damages on the site.”

“Changes in the radiation situation have not been registered,” the State Nuclear Regulatory Inspectorate of Ukraine added.

Although damage to the containment shells around the reactors would clearly be of concern, those structures are built to withstand even plane crashes. The biggest worry right now is that something, such as a catastrophic loss of power, could hamper the work of the plant’s cooling systems.

The Ukrainian regulator was blunt about what the loss of cooling ability would mean, saying it would “lead to significant radioactive releases into the environment” that could “exceed all previous accidents at nuclear power plants, including the Chornobyl accident and the accident at the Fukushima Daiichi nuclear power plant.” (Chornobyl is the Ukrainian spelling of the plant commonly known by its Russian name, Chernobyl.)

That’s not the only worry, though: Zaporizhzhia’s grounds also house a spent nuclear fuel storage facility, which, if damaged, “will also lead to radioactive releases,” the agency said.

The International Atomic Energy Agency (IAEA) has put its incident and emergency center in “full response mode,” and the U.S. Department of Energy has activated its nuclear incident response team.

A week ago, Russian forces seized Chernobyl, which in 1986 suffered the world’s worst nuclear disaster thus far. Some radiation spikes were noted, probably owing to radioactive soil at the disaster site being churned up by military vehicles, whose occupants would probably be the worst affected.

In a Friday morning press conference, IAEA chief Rafael Mariano Grossi said he had asked the Ukrainian and Russian sides to personally visit Chernobyl for negotiations around ongoing nuclear safety in Ukraine. Neither side has yet given the go-ahead, but Grossi said he hoped to undertake the trip after he visits Tehran for a planned visit in the coming days.

Grossi said the situation was unprecedented, and, while his agency would normally engage with a country’s presidency, the IAEA needs “to do something about what is going on, and not simply to tweet and say things from Vienna.”

“The situation [regarding nuclear safety in Ukraine] is very difficult to sustain. What happened last night is proof of that,” he said. “We should not wait for something like this to happen before trying to address it in a more efficient way.”

This article was updated to include Grossi’s comments.