Russian attacks on Ukraine’s power grid are endangering its nuclear plants, a U.N. agency warns.

KYIV, Ukraine — Russia’s assault on Ukraine’s energy grid is endangering the safe operation of the country’s nuclear power plants, the United Nations nuclear watchdog agency has warned.

When Russian missiles knocked one plant offline in Khmelnytskyi in western Ukraine on Tuesday evening, it was left it to rely on diesel generators to perform critical cooling functions. A second nuclear power plant, in nearby Rivne, was forced to reduce the energy it produces after Russian strikes on Tuesday damaged the power lines that connect it to the national grid.

And in southern Ukraine, the Russian occupation of the Zaporizhzhia power plant has forced engineers there to cycle down all of its reactors. For months, attacks in and around that plant have left engineers racing from one crisis to the next as it has repeatedly been disconnected from the national energy grid.

The effect of the new strikes on the energy grid is “a very concerning development,” Rafael M. Grossi, the director of the U.N. group, the International Atomic Energy Agency, said in a statement.

He said it showed that all of Ukraine’s nuclear facilities are vulnerable.

Ukraine relies on its three working nuclear power plants — Khmelnytskyi, Rivne and South Ukraine — to generate much of the power used in the country. It is now struggling to keep open these Ukrainian-controlled plants amid Russia’s assault on the energy grid.

It also still needs to supply energy to two other nuclear plants: the one at Chernobyl, where a meltdown in 1986 caused the world’s worst nuclear disaster, and which still needs energy to run cooling equipment; and the plant in Zaporizhzhia, which is now controlled by Russia and, despite being offline, still needs cooling equipment running around the clock.

At the Khmelnytskyi plant, when missile attacks severed its connection to the grid, its four operating power lines were lost over the course of two and a half hours, the nuclear agency said. It took nine hours to repair some of the damaged lines and reconnect the plant to the grid, and the two reactors remained offline as of Thursday morning, halting their delivery of electricity to hundreds of thousands of households, factories and other customers.

The plant in Rivne lost its connection to one high-voltage power line on Tuesday afternoon, prompting it to reduce power output.

Tuesday’s missiles assault on Ukraine’s energy grid was the largest bombardment of critical infrastructure since the war began. It continued on Thursday, part of a coordinated campaign by Moscow to plunge Ukraine into darkness, make cities unlivable and prompt another wave of people to leave the country.

The strikes on energy infrastructure affect virtually every part of daily life, including central heating systems, clean water supplies, sewage systems and telecommunications. Before the war began nine months ago, Ukraine’s nuclear power plants supplied more than half of the nation’s energy.

Since then, Russia has made 126 attacks on Ukraine’s energy system, including 92 in October and November as winter looms, according to the Ukrainian prosecutor’s office. Local officials in Dnipro in central Ukraine and in Odesa in the south reported further attacks on Thursday.