ZAPORIZHZHIA, Ukraine — The main front in Russia’s military onslaught on Ukraine appears to have shifted dangerously to the south of the country, risking a catastrophe at Europe’s largest nuclear power plant and setting up a possible make-or-break struggle for an important regional capital seized by Russia at the start of its invasion.
Initially focused on the north around the capital, Kyiv, and then turning into a brutal slugfest in the east involving months of artillery duels that cost thousands of lives on both sides, the war has entered a new and, each side hopes, decisive phase.
With fighting raging around the Zaporizhzhia nuclear station and the Russian-occupied city of Kherson, around 60 miles down the Dnipro River from the nuclear plant, the south is now where both Russia and Ukraine are focusing their firepower — and their hopes of avoiding a stalemate that could drag on for years.
Over the weekend, Russia used territory around the nuclear power station, which it seized from Ukraine in March, as a staging ground for attacks on Ukrainian positions. It unleashed a barrage of howitzer fire on the nearby Ukrainian-held town of Nikopol, local officials said.
The intensifying battles around the power plant, which have sent residents in the area fleeing and stirred alarm of a radiation risk far beyond Ukraine, came as Russian forces in Kherson faced encirclement by the Ukrainian military.
The precarious position of the Russian troops in Kherson, who were largely cut off from their main source of supplies after Ukraine wrecked the last of four bridges across the Dnipro, has led to speculation about their fate.
Some reports on Saturday said Russian commanders had already retreated from the city. A regional legislator, Serhiy Khlan, told Ukrainian television on Sunday that Russia is moving its Kherson command center across the Dnipro to more secure territory on the eastern bank.
Senior Ukrainian military sources, however, said they had seen no evidence that Russian commanders were retreating. Analysts warned that Ukrainian politicians have an interest in exaggerating Russia’s troubles to rally morale and demoralize Russian troops.
But Russian forces in Kherson clearly face difficulties now that their supply lines have been compromised. And Mr. Khlan said the only way they could now reach territory securely held by Russia on the east side of the Dnipro, was to use pontoon bridges or to cross by foot, without their equipment, across badly damaged bridges.
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An all-out offensive on Kherson, long threatened by Ukraine but so far limited to attacks on nearby villages and warnings to Russian troops stationed there, has made Moscow particularly eager to hang on to the Zaporizhzhia Nuclear Plant up the Dnipro River to the northeast.
Its objective is to pin down Ukrainian troops and prevent them from joining any battle for Kherson, the first major city seized by Russia at the start of the war, and shells fired from the direction of the nuclear facility have streaked into a small nearby town held by Ukraine across a reservoir on the Dnipro.
In a Saturday night address to his country, the Ukrainian president, Volodymyr Zelensky, accused Moscow of “nuclear blackmail” and warned Russian soldiers at the Zaporizhzhia plant that they had become “a special target” for Ukraine’s special services and military.
But the Ukrainian military has said it has limited options. It worries that if its forces fire back at the Russians, they might hit the sprawling Zaporizhzhia facility, the first active nuclear power plant in a combat zone.
As the fighting rages near active reactors and stored nuclear waste, the United States and the European Union have called for the creation of a demilitarized zone around the plant. But residents fearful of a nuclear disaster are already rushing to get out of the area, and things are becoming increasingly dire, Dmytro Orlov, the exiled mayor of one nearby city, Enerhodar, said in an interview on Sunday.
“With every day the situation gets worse,” he said.
Mr. Orlov said he had spoken with a shift boss at the nuclear plant on Sunday morning, and was told that the whistle and thud of artillery fire were now heard nearly continuously there. “They are shooting day and night,” Mr. Orlov said. The shift boss intended to drive his family out of the area and was considering not returning, Mr. Orlov said.
Since Russia’s capture of the plant in March, its army has controlled the facility, while Ukrainian engineers have continued to operate it. Plant employees, Mr. Orlov said, are “trying to get their wives and children out” while still holding down shifts at the nuclear site. An exodus by Ukrainian workers at the site would raise the risk that, even if not hit by missiles, the reactors could malfunction without trained staff to watch over them.
Fighting has also intensified in Russian-occupied Enerhodar, which is about three miles from the nuclear plant. Russian troops are firing grad rocket artillery from the town’s outskirts, Mr. Orlov said.
Energoatom, a state company that operates Ukraine’s nuclear power stations, said in a statement Sunday that one person had been killed by Russian shelling near Enerhodar and that two workers from the nuclear plant had been wounded.
Russia also kept up pressure around the city of Donetsk to the north, with proxy militias under the so-called Donetsk People’s Republic gnawing away at Ukrainian positions along a front line largely frozen since 2014, when Russia first invaded eastern Ukraine and seized Crimea.
Russia’s defense ministry said Saturday that its forces had taken full control of Pisky, a village on the outskirts of Donetsk, but Britain’s defense intelligence agency said late Saturday that the village “probably remains contested.”
Russia’s main goal over the past week, the British agency said, “has likely been to reorientate units to reinforce southern Ukraine.”
After months of seesawing combat and incremental gains by Russia in the Donetsk region and other parts of eastern Ukraine, Russia has now shifted its energies to the south, where any major gains by Ukraine would threaten Russia’s grip on Crimea, the Black Sea peninsula it seized in 2014. In what Ukraine warned was a taste of things to come for Russia’s military, at least eight Russian warplanes were wrecked last week in a series of explosions at a Russian air base in Crimea.
Overnight and into Sunday morning, Russian howitzers fired on the Ukrainian town of Nikopol, Yevheny Yetushenko, the Ukrainian military governor of the town, said in a post on Telegram.
Russia denies trying to draw Ukrainian fire on the plant and says Ukraine alone is responsible for any damage to the facility, which comprises six separate nuclear reactors.
Former employees who fled say that they saw Russian military equipment on the territory of the plant and that Russia has been assembling heavy weapons there for several weeks.
As conditions unraveled in and near the plant and the flight of civilians from the area accelerated, long lines of cars carrying fleeing civilians formed around Enerhodar over the weekend.
“Locals are abandoning the town,” said a former engineer, who asked to be identified by only his first name, Oleksiy, because of security concerns.
Engineers say that yard-thick reinforced concrete containment structures protect the reactors from even direct hits. International concern, however, has grown that shelling could cause a fire or cause other damage that would lead to a nuclear accident.
The six pressurized water reactors at the complex retain most sources of radiation, reducing risks. After pressurized water reactors failed at the Fukushima nuclear plant in Japan in 2011, Ukraine upgraded the Zaporizhzhia site to enable a shutdown even after the loss of cooling water from outside the containment structures, Dmytro Gortenko, a former plant engineer, said in an interview.
Most Ukrainian employees are not fleeing but they are sending their families away, said Oleksiy, who left in June. Enerhodar was built for plant employees in the Soviet period and had a prewar population of about 50,000.
Michael Schwirtz contributed reporting from Odesa, Ukraine, and Marc Santora from Kyiv.