Russia and Ukraine trade accusations of a potential attack on a nuclear complex, terrifying its workers.

KYIV, Ukraine — Tensions around the nuclear power plant on the front lines of the war in Ukraine escalated sharply on Thursday, as the Russian and Ukrainian militaries traded charges that each was preparing to stage an attack on the plant in coming days, risking a catastrophic release of radiation.

The Russian Ministry of Defense claimed that the Ukrainian military was preparing a “terrorist attack” on the sprawling Zaporizhzhia power plant complex on the Dnipro river, prompting the Ukrainian military intelligence agency to respond that the Russian warning was in fact a pretext for Moscow to stage a “provocation” of some kind there on Friday.

Invading Russian forces seized control of the plant, Europe’s largest nuclear power station, in early March and put it under the control of Rosatom, the Russian state nuclear company. But they have kept Ukrainian staff there — many of them by force, Ukraine claims — to operate it. On Thursday, the Ukrainian intelligence agency said that Rosatom engineers had “urgently” left the plant and that only “operative personnel” would be allowed at the plant on Friday.

“Entrance for all other employees will be closed,” it said.

An employee at the plant, communicating to The New York Times through a colleague in Kyiv, said that workers were terrified.

“The situation is terrible, everyone is scared of tomorrow’s provocations announced by Russia,” said the person, who spoke on the condition of anonymity out of fear for her safety. “Russia’s Defense Ministry said they expect provocations from the Ukrainian side, but we understand perfectly well what that means. Even more people are trying to leave.”

A Telegram post from workers at the plant urged the combatants to consider the danger facing its 10,000 workers and warned that a disaster at the plant could be worse than any the world has ever seen, including at Chernobyl and Fukushima:

“Think about the future of our Earth, about the future of our and your children! Our planet is so small and it is illogical to assume that it will be possible to hide somewhere from the consequences of a significant nuclear catastrophe.”

The fears of a possible attack came even as the United Nations secretary general, António Guterres, held talks in the city of Lviv, in western Ukraine, with the Ukrainian and Turkish presidents in an effort to ease military tensions around the site and restart diplomatic engagement to end the nearly six-month-old war. U.N. nuclear regulators have been pleading for access to the plant to ensure its safety. The United Nations has offered to send nuclear inspectors to the site, if both sides agree.

In recent weeks, daily shelling at the Zaporizhzhia complex has sparked international concerns about the possibility of a meltdown or release of radiation from spent nuclear fuel in storage. Each side has blamed the other for the shelling on the plant, which is Europe’s largest nuclear power complex, with six reactors on a site occupying about half a square mile.

The uncertainty surrounding the security of the plant was underscored by a video that circulated online on Thursday, showing at least five apparent military trucks inside one of the complex’s buildings. By using archival photos of the interior of the different facilities on the premises of the nuclear complex, The Times determined that the videographer walked along a turbine inside the machine room of one of the complex’s six nuclear reactors. Earlier this month, Ukraine released aerial surveillance footage showing several military vehicles driving into one of these buildings.

Why Russia would place military vehicles so close to a reactor is unclear. The reactors themselves are heavily fortified.

The Russian Foreign Ministry on Thursday rejected international calls to have its forces pull back from the facility and create a demilitarized zone around the plant.

Ukraine’s president, Volodymyr Zelensky, issued a strongly worded alarm on Thursday. “The world is on a verge of nuclear disaster due to occupation of world’s third largest nuclear power plant,” he tweeted, adding, “How long will it take the global community to respond to Russia’s irresponsible actions and nuclear blackmailing?”