Ukraine on Sunday accused Russian troops of war crimes after the discovery of mass graves and civilians apparently "executed" in the streets of Bucha, near the capital Kyiv.
The claims came as explosions rocked the Black Sea port city of Odessa, which has largely been spared in the conflict, with air strikes apparently targeting key infrastructure.
In Bucha, AFP reporters saw at least 20 bodies, all in civilian clothing, strewn across a single street. One had his hands tied behind his back with a white cloth, and his Ukrainian passport left open beside his body.
"All these people were shot," Bucha's mayor Anatoly Fedoruk said, adding that 280 other bodies had been buried in mass graves elsewhere in Bucha.
Ukraine's Foreign Minister Dmytro Kuleba called it a "deliberate massacre" and urged G7 countries to impose "devastating" sanctions immediately.
"It looks exactly like war crimes," President Volodymyr Zelensky's spokesman told BBC television.
"We found mass graves. We found people with their hands and with their legs tied up... and with shots, bullet holes, in the back of their head.
"They were clearly civilians and they were executed."
UK Foreign Secretary Liz Truss called for "indiscriminate" Russian attacks in Bucha and elsewhere to "be investigated as war crimes."
"We will not allow Russia to cover up their involvement in these atrocities through cynical disinformation," she added.
In Brussels, European Council chief Charles Michel said the EU was helping Ukraine and NGOs gather evidence "for pursuit in international courts."
And Germany's vice chancellor and economy minister Robert Habeck said a "terrible war crime" had been carried out in Bucha and called for fresh EU sanctions against Russia.
The International Criminal Court in The Hague has already opened a probe into possible war crimes committed in Ukraine.
Several Western leaders, including U.S. President Joe Biden, have accused Russia's Vladimir Putin of being a "war criminal."
Human Rights Watch said it had documented cases of Russian troops committing possible war crimes against civilians in occupied areas of Chernigiv, Kharkiv, and Kyiv, including rape and summary execution.
Zelensky has also alleged that Russian soldiers planted mines and other booby traps as they withdraw from northern Ukraine, warning returning residents to be wary of tripwires and other dangers.
Ukraine's deputy prime minister Iryna Vereshchuk meanwhile said 11 local community leaders in Kyiv, Kherson, Kharkiv, Zaporizhzhia, Mykolaiv and Donetsk were "in captivity."
In Odessa, plumes of thick black smoke billowed over the strategic port city, after air strikes shook residents awake at about 6:00 a.m. (03:00 GMT).
"We were woken up by the first explosion then we saw a flash in the sky, then another, then another. I lost count," one local man, Mykola, 22, told AFP from the roof of a building overlooking the site.
Russia's Defense Ministry said, "High-precision sea and air-based missiles destroyed an oil refinery and three storage facilities for fuel and lubricants" at Odessa that were supplying fuel to Ukrainian troops.
Anton Herashchenko, an adviser to Ukraine's interior minister, said: "Some of the missiles were shot down by air defense."
The strikes came as top UN humanitarian envoy Martin Griffiths was expected in Moscow then Kyiv to seek a halt in the fighting, which by Ukrainian estimates has left some 20,000 people dead.
Nearly 4.2 million Ukrainians have fled the country since Russia invaded on Feb. 24, with almost 40,000 pouring into neighboring countries in the last 24 hours alone, the UN refugee agency said.
The International Organization for Migration said nearly 6.48 million were estimated to be displaced inside Ukraine.
Pope Francis, on a visit to the Mediterranean island of Malta on Sunday, made a plea for refugees fleeing the "sacrilegious war" in "tormented Ukraine" to be welcomed.
On talks to end the fighting, Russia's chief negotiator Vladimir Medinsky said it was too early for a top-level meeting between Zelensky and Putin on ending the conflict.
He said Ukraine had become "more realistic" in its approach to issues related to the neutral and non-nuclear status of Ukraine but a draft agreement for submission to a summit meeting was not ready.
He said he did not share the "optimism" of Ukraine's negotiators on the possibility of talks between the two countries' leaders in Turkey.
His Ukrainian counterpart, David Arakhamia, had said on Saturday that Moscow had "verbally" agreed to key Ukrainian proposals, raising hopes that talks to end fighting were moving forward.
Ukraine has proposed abandoning its aspirations to join NATO and declaring official neutrality, if it obtains security guarantees from Western countries. It would also pledge not to host any foreign military bases.
It has proposed to temporarily put aside the question of Crimea, annexed by Russia in 2014, and two breakaway territories in the eastern Donbas region that Russia has recognized as independent.
Medinsky said Russia's position on Crimea and the Donbas "remains unchanged" and that talks would resume by video conference on Monday.
As Russian forces withdraw from some northern areas, Moscow appears to be focusing on eastern and southern Ukraine, where it already holds vast swathes of territory.
UK Defense Intelligence said early Sunday that Russian air activity in the last week had been concentrating on southeastern Ukraine, "likely as a result of Russia focusing its military operations in this area."
But it said Russia was struggling to find and destroy air systems, which has "signficantly affected their ability to support the advance of their ground forces."
In his latest video message, Zelensky said Russian troops wanted to seize the disputed Donbas region and the south of Ukraine, promising "to defend our freedom, our land and our people."
Ukraine on Saturday claimed progress against Russian forces, saying Irpin, Bucha, Gostomel and the whole Kyiv region had been "liberated."
Russia's efforts to consolidate its hold on southern and eastern areas of Ukraine have been hampered by the resistance of Mariupol despite devastating attacks lasting weeks.
At least 5,000 residents have been killed in the besieged southern port city, according to officials, while the estimated 160,000 who remain face shortages of food, water and electricity.
The International Committee of the Red Cross (ICRC) said its team left for Mariupol on Saturday to make another attempt at conducting an evacuation, after being forced to turn back the day before.