(NewsNation) — Mandatory evacuations have been issued in east Ukraine after Ukraine and Russia traded accusations over an attack early Friday that appeared to have killed dozens of Ukrainian prisoners of war in Olenivka.

Ukrainian President Volodymyr Zelenskyy said on Saturday his government was ordering the mandatory evacuation of people in the Eastern Donetsk region, a scene of fierce fighting with Russia.

In a late-night television address, Zelenskyy said the hundreds of thousands of people, still in combat zones in the larger Donbas region, which contains Donetsk as well as the neighboring Luhansk region, needed to leave.

“The more people leave (the) Donetsk region now, the fewer people the Russian army will have time to kill,” he said, adding that residents who left would be given compensation.

Separately, domestic Ukrainian media outlets quoted Deputy Prime Minister Iryna Vereshchuk as saying the evacuation needed to take place before winter begins, since the region’s natural gas supplies had been destroyed.

Zelenskyy said hundreds of thousands of people were still living in areas of Donbas where fighting was fierce.

“Many refuse to leave but it still needs to be done,” the president said. “If you have the opportunity, please talk to those who still remain in the combat zones in Donbas. Please convince them that it is necessary to leave.”

Earlier on Saturday, Ukraine’s military said more than 100 Russian soldiers had been killed and seven tanks destroyed in fighting in the south on Friday, including the Kherson region that is the focus of Kyiv’s counteroffensive in that part of the country and a key link in Moscow’s supply lines.

Rail traffic to Kherson over the Dnipro River had been cut, the military’s southern command said, potentially further isolating Russian forces west of the river from supplies in occupied Crimea and the east.

South of the town of Bakhmut, which Russia has cited as a prime target in Donetsk, the Ukrainian military said Russian forces had been “partially successful” in establishing control over the settlement of Semyhirya by storming it from three directions.

“He established himself on the outskirts of the settlement,” the military’s evening report said, referring to Russian forces.

Defence and intelligence officials from Britain, which has been one of Ukraine’s staunchest allies since Moscow invaded its neighbor on Feb. 24, portrayed Russian forces as struggling to maintain momentum.

Ukraine has used Western-supplied long-range missile systems to badly damage three bridges across the Dnipro in recent weeks, cutting off Kherson city and – in the assessment of British defense officials – leaving Russia’s 49th Army highly vulnerable on the river’s west bank.

The Kherson region’s pro-Ukrainian governor, Dmytro Butriy, said fighting was continuing in many parts of the region, and that Berislav district, just northwest of the Kakhovka hydroelectric plant, was particularly hard hit.

“In some villages, not a single home has been left intact, all infrastructure has been destroyed, people are living in cellars,” he wrote on Telegram.

Just to the north of Lysychansk, which Moscow’s forces captured in early July after weeks of fighting, Ukrainian partisans destroyed a railway junction box near the Russian-controlled town of Svatove on Friday night, making it harder for Moscow to transport ammunition to the front lines by train, Luhansk regional governor Serhiy Gaidai said in an online post.

Reuters could not independently verify the battlefield reports.

Officials from the Russian-appointed administration running the Kherson region earlier this week rejected Western and Ukrainian assessments of the situation.

On Friday the British ministry described the Russian government as “growing desperate”, having lost tens of thousands of soldiers in the war. British MI6 foreign intelligence agency chief Richard Moore added on Twitter that Russia is “running out of steam.”

Prison Deaths

Ukrainian and Russian officials blamed each other Saturday for the deaths of dozens of Ukrainian prisoners of war in an attack on a prison in a separatist-controlled area. The International Red Cross asked to visit the prison to make sure the scores of wounded POWs had proper treatment, but said its request had not been granted so far.

Meanwhile, Russia kept launching attacks on several Ukrainian cities, hitting a school and a bus station.

Ukrainian President Volodymyr Zelenskyy said the ICRC and the United Nations have a duty to react to the shelling of the prison complex in Ukraine’s eastern Donetsk province, and he called again for Russia to be declared a terrorist state.

“Condemnation at the level of political rhetoric is not enough for this mass murder,” he said.

Separatist authorities and Russian officials said the attack Friday killed 53 Ukrainian POWs and wounded another 75. Russia’s Defense Ministry on Saturday issued a list naming 48 Ukrainian fighters, aged 20 to 62, who died in the attack; it was not clear if the ministry had revised its fatality count.

Satellite photos taken before and after the attack show that a small, squarish building in the middle of the Olenivka prison complex was demolished, its roof in splinters.

Both Ukraine and Russia alleged the attack on the prison was premeditated and intended to silence the Ukrainian prisoners and destroy evidence.

The ICRC, which has organized civilian evacuations and worked to monitor the treatment of POWs held by Russia and Ukraine, said it requested access to the prison “to determine the health and condition of all the people present on-site at the time of the attack.”

“Our priority right now is making sure that the wounded receive lifesaving treatment and that the bodies of those who lost their lives are dealt with in a dignified manner,” the Red Cross said.

But the organization said late Saturday that its request to access the prison had not been granted yet.

“Granting ICRC access to POWs is an obligation of parties to conflict under the Geneva Conventions,” the ICRC said on Twitter.

Russia claimed Ukraine’s military used U.S.-supplied precision rocket launchers to target the prison in Olenivka, a settlement controlled by the Moscow-backed Donetsk People’s Republic.

The Ukrainian military accused the Russians of shelling the prison to cover up the alleged torture and execution of Ukrainians there.

The Institute for the Study of War, a think tank based in Washington, said the competing claims and limited information prevented assigning full responsibility for the attack but the “available visual evidence appears to support the Ukrainian claim more than the Russian.”

Moscow has opened a probe into the attack and the U.N. said it also was prepared to send investigators. U.N. deputy spokesman Farhan Haq said “we stand ready to send a group of experts able to conduct an investigation, requiring the consent of the parties, and we fully support the initiatives” of the Red Cross.

Elsewhere in eastern Ukraine, Russian rockets hit a school in Kharkiv and a bus station in Sloviansk, among other strikes. In southern Ukraine, one person was reported killed and six injured in shelling in a residential area in Mykolaiv, local officials said.

Russian and separatist forces are trying to take full control of the Donetsk region, one of two eastern provinces that Russia has recognized as sovereign states.

Deputy Prime Minister Iryna Vereshchuk warned Saturday that Ukrainian-controlled parts of Donetsk will face severe heating problems this winter because of the destruction of gas mains. She called for a mandatory evacuation of residents before the cold weather sets in.

The prison attack reportedly killed Ukrainian soldiers captured in May after the fall of Mariupol, a Black Sea port city where troops and the Azov Regiment of the national guard famously held out against a months-long Russian siege.

On Saturday, an association of Azov fighters’ relatives dressed in black demonstrated outside Kyiv’s St. Sophia Cathedral and called for Russia to be designated a terrorist state for violating the Geneva Convention’s rules for the treatment of war prisoners.

A woman wearing dark glasses who gave only her first name, Iryna, was waiting for news of her 23-year-old son.

“I don`t know how is he, where is he, if he is alive or no. I don`t know. It`s a horror, only horror,” she said.

On the energy front, Russia’s state-owned natural gas corporation said Saturday it has halted shipments to Latvia because of contract violations. Gas giant Gazprom said the shipments were stopped because Latvia broke “terms for extraction of gas.”

The statement likely referred to a refusal to meet Russia’s demand for gas payments in rubles rather than other currencies. Gazprom has previously suspended gas shipments to other EU countries, including the Netherlands, Poland and Bulgaria, because they would not pay in rubles.

EU nations have been scrambling to secure other energy sources, fearing that Russia will cut off more gas supplies as winter approaches.

Reuters contributed to this report.

The Associated Press contributed to this report.