Authorities found the bodies of dozens of civilians in a mass grave in Buzova, a Ukrainian village west of Kyiv, following the Russian troops' withdrawal.
Local leader Taras Didych told Ukrainian TV that the bodies were found in a pit near a petrol station.
Bodies had also been found in a dozen shelled cars on the main road from Kyiv to Zhytomyr, which goes through Buzova, he said.
Russian troops recently withdrew from the area around Kyiv after spending the first few weeks of the war trying unsuccessfully to blockade the city.
The United Nations has been able to confirm more than 1,700 civilian deaths. But, like the government in Kyiv, the U.N. assumes the actual toll is far higher.
Ukrainian Prosecutor-General Iryna Venediktova accused Russia of committing war crimes and crimes against humanity in all regions of the country, and called Russian President Vladimir Putin the "main war criminal of the 21st century."
Ukraine identified 5,600 cases of alleged war crimes with 500 suspects, she said.
Meanwhile, Ukrainian Deputy Defence Minister Hanna Malyar says Russia continued to work on its minimum goal of taking eastern Ukraine, according to the UNIAN news agency.
The general staff of the Ukrainian army expects a new push by the Russian forces to completely take over eastern Ukraine, with new troops from other parts of Russia currently being brought to the borders.
The focus of the next Russian attacks is expected to be near Kharkiv in the north-east and Sloviansk in Donetsk Oblast, according to the generals as reported by UNIAN.
The governor of the embattled Luhansk region also said he assumes that the Russians will soon launch a major offensive in eastern Ukraine.
"It is a matter of days," Serhiy Hayday told the Italian newspaper Corriere della Sera. "They are repositioning themselves on the border and continue to bomb us. They don't know morality any more: They raze hospitals, schools and houses to the ground."
Moscow-backed separatists have controlled parts of the southeastern Ukrainian regions of Donetsk and Luhansk, known collectively as the Donbass, for almost eight years.
Putin recognized the self-proclaimed "people's republics" of Luhansk and Donetsk as independent states shortly before invading Ukraine.
Asked what lay ahead now, Hayday said: "Hell." He referenced Bucha and Mariupol, where horrific attacks and war crimes have been observed for weeks.
"In our area, it is getting much worse," the governor said. Unlike in other parts of the country, he said, there are hardly any bunkers left in Luhansk for Ukrainians to seek shelter.
"We are hiding in the basements. I am trying to convince all my fellow citizens to leave here," he told the newspaper.
In the Donetsk region at least five civilians have been killed and five others injured in the shelling, according to the local military administration.
At least two other people were killed after Russian troops shelled settlements in Kharkiv, Ukrainian authorities said.
Elsewhere, the airport of the Ukrainian city of Dnipro, a major industrial hub, was destroyed by Russian rocket fire, according to local administrator Valentin Reznichenko. He said that efforts were under way to count the victims.
The Russian forces are seeking to boost their troop levels with personnel who have been discharged from military service since 2012, the British Defense Ministry said on Sunday in its regular intelligence update.
Efforts to gain more combat power include trying to recruit forces in the Russian separatist-controlled Transnistria region of Moldova, it said.
It is unclear how many Russian soldiers have been killed since the invasion of Ukraine began on Feb. 24. A Kremlin spokesperson conceded on Thursday that Russia had suffered a "significant" number of troop deaths.
In the West, according to the BBC, it is assumed that between 7,000 and 15,000 Russian soldiers have been killed so far.
Austrian Chancellor Karl Nehammer will meet Putin in Moscow on Monday afternoon, government spokesperson Daniel Kosak confirmed to Deutsche Presse-Agentur (dpa).
Nehammer will fly to Moscow via Turkey and will be the first Western leader to meet Putin in Russia since the start of the war on Feb. 24. Peskov also confirmed the meeting.
The chancellor plans to discuss the alleged war crimes committed by Russia with the head of the Kremlin and also talk about arrangements for humanitarian corridors from eastern Ukraine, where a "big battle" is expected in the coming days.
According to sources close to Nehammer, he is acting in coordination with Ukrainian President Volodymyr Zelenskyy, European Commission President Ursula von der Leyen and German Chancellor Olaf Scholz.
On Saturday, Nehammer met Zelenskyy in Kyiv.
The Ukrainian president is not expected to meet Putin any time soon for peace talks, presidential adviser Mykhailo Podolyak said on Ukrainian TV.
Ukraine continues to insist on strong security guarantees and is paying a very high price for them, Podolyak said. "Yes, it is hard, we are losing people and infrastructure every day. But Russia must give up its imperial illusions," he said.
Zelenskyy said Russia can't admit the mistakes it has made in its policy towards Ukraine over the years.
"They are afraid to admit that for decades, they have taken wrong positions and spent colossal resources to support human zeros they wanted to build up as future heroes of Ukrainian-Russian friendship," Zelenskyy said in his daily video address on Sunday evening.
In addition, he said, Russia is trying to shift the blame for everything onto Ukraine. "They seized Crimea, we are supposedly to blame for that," said Zelenskyy. "They have destroyed all normal life in Donbass, we are allegedly to blame for that. They've been killing people in our country for eight years (and) they say it's our fault."
"And finally they started a full-scale war against us, and again it's our fault." And all this out of "pure cowardice," Zelenskyy summed up.
Meanwhile, the number of people who have fled the war in Ukraine has exceeded 4.5 million, according to data from the U.N. refugee agency./DPA