Ukrainian Defence Minister Oleksii Reznikov has accused Russian troops of massacring civilians in the town of Bucha, where as many as 340 bodies have been recovered, according to Ukrainian media reports.
On Facebook, Reznikov threatened retaliation. "This evil simply cannot go unpunished," he said on Monday.
"Our intelligence is consistently identifying all invaders and killers," Reznikov wrote, adding that each of them will get what they "deserve" at their own time.
Images of residents' bodies, which were discovered after Russian troops withdrew from the area near Kiev, have prompted international horror.
Ukrainian President Volodymyr Zelensky travelled to Bucha on Monday to see at first hand the death and destruction.
War crimes had been committed in Bucha, Zelensky told journalists. "The world will recognize this as genocide," he said.
Zelensky was accompanied by security guards and wore a protective vest as he took stock of the devastation. Bodies and burnt out Russian tanks littered the streets.
When asked by a reporter whether it was still possible to negotiate peace with Russia, he answered in the affirmative: "Ukraine must have peace."
Reznikov compared the actions of the Russian army to those of the Nazi Schutzstaffel, a major paramilitary organization under Adolf Hitler, during the World War II.
Moscow denies the charges. "We categorically reject all accusations," Kremlin spokesperson Dmitry Peskov said on Monday, according to the Interfax agency.
Russian Foreign Minister Sergei Lavrov said the country's ambassador to the UN in New York would hold a press conference on Monday evening addressing the issue.
Lavrov said evidence would be presented to show the "true nature of those events in Bucha, that our Western partners are trying to interpret as testimonies of war crimes committed by the Russian Federation."
Lavrov added that negotiations between the Russian and Ukrainian sides for an end to the fighting were continuing "intensively." He called on the government in Kiev to be guided by its own national interests. Ukraine should not listen to its advisers abroad, Lavrov said, referring to the US.
Meanwhile Russia's Investigative Committee has begun proceedings into what it calls the spread of "false reports" about killings of civilians in Bucha.
Russia's chief investigator Alexander Bastrykin called for an assessment of the "provocation on the part of Ukraine," the Investigative Committee said.
Moscow's investigation is not about the incident itself, but about the spread of information about it. Defamation of the Russian army is a punishable offence in the country.
Meanwhile, the EU plans to send investigative teams to Ukraine to look into alleged Russian war crimes in Bucha and elsewhere.
European Commission President Ursula von der Leyen said after a conversation with Zelensky that the EU's judicial authority Eurojust and the law enforcement agency Europol are ready to provide support.
This group is to collect evidence and investigate war crimes and crimes against humanity.
US President Joe Biden has called for Russian President Vladimir Putin to be tried for war crimes. "He should be held accountable," Biden said. "This guy is brutal and what's happening in Bucha is outrageous and everyone's seen it."
He added that it was a "war crime." Investigations must now gather "all the details" so we can "have a war crime trial."
He also said that Washington would continue to tighten sanctions on Russia due to the its war on Ukraine.
French President Emmanuel Macron also called for members of the Russian army to be brought to justice for war crimes linked to the Bucha massacre.
"It is clear that today there is evidence of war crimes, it was the Russian army that was in Bucha," Macron told France Inter. "International justice must take care of this and those who committed these crimes will have to answer for them," Macron said.
On the ground, Ukraine's military said it continues to record Russian artillery attacks on the besieged eastern city of Kharkiv.
There was also a likelihood of air and missile attacks on civilian targets in Kharkiv, the General Staff of the Armed Forces reported.
In addition, Russia had strengthened air defences for its troops in the region and for the city of Belgorod on the Russian side of the border.
A fuel depot had gone up in flames in Belgorod last week. Russia said Ukraine carried out a helicopter attack on the facility.
Meanwhile Russia has withdrawn about two-thirds of its troops around Kiev, according to a senior US defence official.
The remaining soldiers continue to be positioned outside the Ukrainian capital, and it is open to question whether and when they would also withdraw northward, said the official who spoke on condition of anonymity.
"We continue to believe is that they're going to be refit, resupplied, perhaps maybe even reinforced with additional manpower, and then sent back into Ukraine to continue fighting elsewhere," he says, adding they will likely be sent to the Donbass region.
