Ukrainian President Volodymyr Zelensky said on Saturday his government was ordering the mandatory evacuation of people in the eastern Donetsk region, scene of fierce fighting with Russia.
In a late-night television address, Zelensky also 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.
Zelensky 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."
It is not the first time Ukrainian authorities have called for civilians to evacuate areas they control in Donetsk, and John Herbst, a former US ambassador to Ukraine, told Reuters it could be due to expectations of heavier fighting rather than fuel shortages.
"I don’t know why Zelensky issued the call," he said. "What I do know is that there has been fierce fighting in Donetsk. The Russians took (neighbouring) Luhansk (oblast) several weeks ago. I expect further fierce fighting in Donetsk."
Herbst said he did not expect Russia to capture the rest of Donetsk given the longer logistics lines they would need and the Ukrainian forces’ use of advanced long-range artillery and rocket systems provided by the United States and others.
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 defence 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."
Ukraine and Russia have traded accusations over a missile strike or explosion early on Friday that appeared to have killed dozens of Ukrainian prisoners of war in the front-line town of Olenivka, held by Moscow-backed separatists in eastern Donetsk.
Russia's defense ministry on Saturday published a list of 50 Ukrainian prisoners of war killed and 73 wounded in what it said was a Ukrainian military strike with a US-made High Mobility Artillery Rocket System (HIMARS).
Ministry spokesman Lieutenant-General Igor Konashenkov said "all political, criminal and moral responsibility" rested with Zelensky, "his criminal regime and Washington who supports them".
The ministry said Russia had invited experts from the United Nations and the Red Cross to probe the deaths "in the interests of conducting an objective investigation".
The separatists put the death toll at 53.
Ukraine's armed forces denied responsibility, saying Russian artillery had targeted the prison to hide mistreatment there. Foreign Minister Dmytro Kuleba said on Friday Russia had committed a war crime and called for international condemnation.
Reuters could not immediately verify the differing versions of events, but some of the deaths were confirmed by Reuters journalists who visited the prison.
The United Nations said earlier it was prepared to send experts to investigate if it obtains consent from both parties. The International Committee of the Red Cross (ICRC) said it was seeking access and had offered to help evacuate the wounded.
US Secretary of State Antony Blinken expressed his condolences in a Friday phone call with Kuleba and said Washington was committed to "hold Russia accountable for atrocities," the US State Department said.
Ukraine has accused Russia of atrocities against civilians and identified more than 10,000 possible war crimes. Russia denies targeting civilians and war crimes.