KYIV, Ukraine — Russia and Ukraine are ramping up their military forces in southern Ukraine amid signs fighting may soon escalate, a U.N. official said on Wednesday, warning that the buildup has further imperiled security at the Zaporizhzhia Nuclear Power Plant, the largest in Europe.
“The situation is not improving,” the head of the U.N.’s nuclear agency, Rafael Mariano Grossi, said, after navigating the front lines to visit the plant on Wednesday. “It is obvious that military activity is increasing in this whole region, so every possible measure and precaution should be taken so that the plant is not attacked and can be protected.”
His comments come just hours after explosions shook Melitopol, a city occupied by Russian forces in the same region as the power plant. Some Ukrainian officials have identified the city as a target for an expected counteroffensive this spring, when Kyiv is likely to push to reclaim land lost to Moscow after its full-scale invasion 13 months ago.
Mr. Grossi, the director general of the International Atomic Energy Agency, crossed the front line on Wednesday from Ukrainian-controlled area into territory controlled by Russian forces to reach the nuclear complex. Russia seized the complex more than a year ago and stationed artillery there to shell nearby towns.
For months, Mr. Grossi has been warning that the shelling around the plant — some of which has hit critical equipment — could lead to a catastrophic nuclear accident. He said in an interview on Tuesday that the situation had no precedent in the history of civilian nuclear power, and in a news conference at the plant on Wednesday, he voiced fresh concerns.
He said that there was “open talk about offensives and counteroffensives,” but gave no further details about the force buildup.
The nuclear complex lies on the eastern bank of the Dnipro River, behind Russian lines. Ukrainian forces are stationed on the western shore of the river across from the plant, and they also hold ground on the east side of the river about 36 miles from the complex.
Ukraine’s military leaders have remained secretive about where they will strike in the expected spring counteroffensive, which would be bolstered by an influx of military aid from the United States and other allies. The Donbas region of eastern Ukraine is also viewed as a possible location.
A Ukrainian counterattack would aim to turn the tables on an offensive launched in the east by Moscow this year. That offensive has set off savage fighting around the city of Bakhmut in Donetsk region, part of the Donbas, and in other towns along the front line. Russian gains have been extremely limited.
But the Zaporizhzhia region also presents an inviting target for the government in Kyiv. Ukrainian forces might seek to push south toward Melitopol and the coast of the Sea of Azov, hoping to cut in half a ribbon of Russian-occupied land connecting Crimea to eastern Ukraine.
Doing so would thwart one of the Kremlin’s military objectives, which was to seize territory along Ukraine’s coast to create a land bridge between the two areas where it has held territory since 2014.
Mykyta Poturaev, a Ukrainian lawmaker, said on Wednesday that attacks in Melitopol formed “part of a preparation operation” before such a push.
The blasts in Melitopol occurred around 5:30 a.m., Vladimir Rogov, a pro-Russian occupation official, said on the Telegram messaging app. A train depot was damaged and parts of the power grid were hit, he said, adding that there were no casualties.
The Zaporizhzhia plant lies around 80 miles northwest of Melitopol. Mr. Grossi has held fruitless talks with President Volodymyr Zelensky of Ukraine and President Vladimir V. Putin of Russia in an attempt to establish a security and safety zone around the complex. Russia has resisted giving up control of the plant.
“It is very very important that we agree on the fundamental principle that a nuclear power plant is not attacked under any circumstances or it shouldn’t be used to attack others,” Mr. Grossi said.
In an implicit rebuke to Russia, he added: “This is a nuclear power plant. It is not a military base. It should never be a military base.”