Russia on Friday said it was sending reinforcements to the Kharkiv region of eastern Ukraine, where Kiev’s forces have announced big gains as part of a broader counteroffensive.

The head of the International Atomic Energy Agency (IAEA), meanwhile, has warned of “dramatic” developments at the Russian-controlled Zaporizhia nuclear power plant in southern Ukraine, where recent fighting has “endangered” the plant’s “safe operation”.

In the Kharkiv region, Ukrainian officials have exhumed two bodies in a village recently retaken from Russia as part of an investigation into a possible war crime.

The village of Grakove was the scene of heavy fighting in the first weeks of the Russian invasion.

Russian state media broadcast footage of armored columns, support vehicles and artillery emblazoned with the letter “Z,” the symbol of the Moscow invasion, navigating asphalt roads and dirt roads in the Kharkiv region.

A Moscow-deployed official, Vitaliy Ganchev, said in TV comments that “bitter fighting” was underway near the town of Balakliya, which Ukraine said it retook on Thursday.

“We don’t control Balakliya. Attempts are being made to drive out Ukrainian forces, but there is fierce fighting and our troops are being held back on the approach,” Ganchev said.

“Now Russian reserves have been brought there, our troops are fighting back,” he added.

IAEA Secretary-General Rafael Grossi warned on Friday that fresh shelling near the Zaporizhia nuclear power plant in the south of the country had caused a power outage in the nearby town of Energodar.

That jeopardized the safe operation of the plant, he said.

“This is totally unacceptable. It can’t stand it,” he said, insisting on “the immediate cessation of shelling throughout the area.”

Petro Kotin, head of Ukraine’s Atomic Energy Agency, told AFP Russian forces tortured nuclear plant workers and at least two people were killed.

“Two people were beaten to death. We don’t know where about ten people are now, they were kidnapped (by the Russians) and after that we have no information about their whereabouts,” he said.

In the recently retaken village of Grakove, where Ukrainian officials exhumed two bodies on Friday, resident Sergiy Lutsay spoke to journalists about what he had seen.

Shortly after the war began, Russian soldiers forced him at gunpoint to bury the bodies, he said.

“They came to my house, I was with my father at the age of 70,” he told journalists. “I was afraid they would threaten him.”

“They told me to come and dig a hole.”

He did not want to confirm a statement by the police that at least one of the two men killed had had their ears cut off.

US Secretary of State Antony Blinken said Russia’s push to reinforce Kharkiv shows Moscow is paying “huge costs” in its attempt to seize and then hold Ukrainian territory.

“There are large numbers of Russian forces in Ukraine and, unfortunately, President (Vladimir) Putin has tragically and horribly shown that he will implicate many people in this matter, at a huge cost to Russia,” he said.

During a visit to Brussels on Friday, he spoke to NATO Secretary General Jens Stoltenberg.

A day earlier, he paid a surprise visit to Kyiv, during which he unveiled another $2.8 billion in military aid and hailed Ukraine’s “clear and real” advances on the frontline.

Late Thursday, President Volodymyr Zelenskyy shared footage showing Ukrainian soldiers in camouflage clothing holding his country’s blue and yellow flag over Balakliya.

The city, which was under Russian control for about six months and had a pre-war population of about 30,000, fell early to Russian troops, who invaded in February.

Now Russian officials in the Kharkiv region say they are evacuating civilians to Russia “until the situation stabilizes.”

On the Ukrainian side, however, Kharkiv regional governor Oleg Synegubov warned people not to return to the newly conquered territories while “cleansing and demining” was taking place.

Zelenskyy said Thursday that the Ukrainian army had recaptured a total of around 1,000 square kilometers (almost 400 square miles) from Russian forces since the beginning of the month.

In the area around the city of Kharkiv, Ukrainian forces pushed 50 kilometers (30 miles) behind Russian lines and recaptured more than 20 towns and villages, military officials said.

The counter-offensive is also making progress in the south of the country, especially in the Kherson region, as well as in Kharkiv and in the industrial province of Donbas in the east.

“It’s very tough, but we’re making progress,” Commander-in-Chief General Valeriy Zaluzhny said on Friday.