Kyiv, Ukraine – Ukrainian President Volodymyr Zelensky has vowed that war criminals will face justice after Defense Minister Oleksii Reznikov accused Russian troops of massacring civilians in the town of Bucha, where as many as 340 bodies have been recovered.

Ukraine was working with the European Union and the International Criminal Court to ensure that war crimes in Bucha and other Ukrainian cities are fully investigated, he said in a video message issued late Monday.

Reznikov threatened retaliation. "This evil simply cannot go unpunished," he said on Facebook on Monday.

"Our intelligence is consistently identifying all invaders and killers," Reznikov wrote, adding that each of them will get what they "deserve" in their own time.

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Images of residents' bodies, which were discovered after Russian troops withdrew from the area near Kyiv, have prompted international outrage.

Zelensky traveled to Bucha on Monday to see firsthand 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 burned out Russian tanks littered the streets.

In New York, Russia's UN Ambassador Vasily Nebenzya dismissed reports of atrocities in Bucha as a "staged provocation."

At a press conference, Nebenzya spoke of "yet another staged provocation aimed at discrediting and dehumanizing of the Russian military."

Kremlin spokesperson Dmitry Peskov also said that Moscow denies the charges. "We categorically reject all accusations," Peskov said on Monday, according to the Interfax news agency.

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Back in Kyiv, when asked by a reporter whether it was still possible to negotiate peace with Russia, Reznikov 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.

Lavrov added that negotiations between the Russian and Ukrainian sides for an end to the fighting were continuing "intensively."

Zelensky has named the members of a Ukrainian delegation that is to discuss security guarantees with Russia. The head of the Ukrainian peace talks delegation, David Arakhamia, is also to lead the talks on security guarantees, according to a decree signed by Zelensky and published late Monday.

During negotiations with Moscow at the end of March, Kyiv had shown a willingness to conclude a treaty on a neutral, non-aligned and nuclear-weapon-free status for Ukraine given certain security guarantees.

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 offense in the country.

Meanwhile, the EU plans to send investigative teams to Ukraine to look into alleged Russian war crimes.

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.

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.

Russian troops were preparing to take the city, Defense Ministry spokesperson Oleksandr Motuzyanyk told the Ukrayinska Pravda newspaper.

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 Kyiv, according to a senior US defense 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 said, adding they will likely be sent to the Donbass region.