KYIV, Ukraine — Russia and Ukraine are increasing their military presence in southern Ukraine amid signs that fighting may soon escalate, a U.N. official said Wednesday, a build-up that has further threatened the security of the Zaporizhzhia nuclear power plant, Europe’s largest.
“The situation is not improving,” said Rafael Mariano Grossi, head of the UN’s nuclear agency, after visiting the front lines to tour the plant on Wednesday. “It is obvious that military activity is increasing in this entire region, so all possible measures and precautions must be taken to protect the plant from attack.”
His comments came hours after an explosion in the city of Melitopol, occupied by Russian forces, in the same area as the power plant. Some Ukrainian officials have identified the city as a target for an expected counteroffensive this spring, as Kyiv may press to regain ground lost to Moscow after its full-scale invasion 13 months ago.
Director General of International Atomic Energy Agency Mr. Grosi crossed the front line from Ukrainian-controlled territory into Russian-held territory on Wednesday to reach the nuclear complex. Russia seized the compound a year ago and stationed artillery to shell nearby towns.
For months, he has been warning that shelling around the plant – some of which have hit critical equipment – could lead to a catastrophic nuclear accident. In an interview on Tuesday, he said the situation was unprecedented in the history of civilian nuclear power, and at a news conference at the plant on Wednesday, he expressed new concerns.
“There was open talk about attacks and counter-attacks,” he said, but did not give further details about the force formation.
The nuclear complex is located on the east bank of the Dnipro River, behind Russian borders. Ukrainian forces are stationed on the west bank of the river across from the plant and have also landed on the east side of the river, 36 miles from the compound.
Ukraine’s military leaders have remained secretive about where they will strike in a spring counteroffensive expected to be bolstered by the arrival of military aid from the United States and other allies. The Donbass region of eastern Ukraine is also seen as a possible location.
The Ukrainian counteroffensive aims to turn the tables on an offensive launched by Moscow in the east this year. The offensive has sparked brutal fighting around the town of Pakmut in the Donetsk region, part of the Donbass and other front-line towns. Russian gains are very limited.
But the Zaporizhzhia region also presents an inviting target for the government in Kiev. Ukrainian forces may seek to push south toward Melitopol and the coast of the Sea of Azov, hoping to cut half of the Russian-occupied ribbon of land connecting Crimea with eastern Ukraine.
Doing so would defeat one of the Kremlin’s military objectives, which is to seize territory along Ukraine’s coast to create a land bridge between the two regions, which have held the territory since 2014.
Ukrainian lawmaker Mykyta Poturaev said on Wednesday that the Melitopol attacks arose “as part of a preparatory operation” before such a push.
The blasts in Melitopol occurred around 5:30 a.m., Vladimir Rokov, a pro-Russian occupation official, said. said on the Telegram messaging app. A railway depot was damaged and parts of the power grid were hit, he said, adding that there was no loss of life.
The Zaporizhzhia plant is 80 miles northwest of Melitopol. President of Ukraine Volodymyr Zelensky and President of Russia Vladimir V. Putin with Mr. Croci held fruitless negotiations. Russia resisted relinquishing control of the plant.
“It is very important that we agree on the basic principle that under no circumstances should a nuclear power plant be attacked or used to attack others,” Mr. Croce said.
In an implicit condemnation of Russia, he added: “This is a nuclear power plant. This is not a military base. It should never be a military base.