The United Nations heard a Russian soldier's final words to his mother sent in a text message before he was killed.
Ukraine's UN Ambassador Sergiy Kyslytsya read the soldier's message where he described "a real war raging" and said "mama this is so hard".
It was read on Monday during an emergency special session on Russia's invasion as it continues its assault on the eastern European country.
The soldier's message read: "Mama I'm in Ukraine. There is a real war raging here. I'm afraid. We are bombing all of the cities together, even targeting civilians.
"We were told that they would welcome us and they are falling under our armoured vehicles, throwing themselves under the wheels and not allowing us to pass.
"They call us fascists. Mama this is so hard."
Click here to follow our live blog with the latest updates
The Russian soldier was said to be responding to his mother's text where she explained that his father was asking if he could send him a parcel.
In her message, she wrote: "How are you doing? Why has it been so long since you responded? Are you really in training exercises?"
The soldier explained that he was no longer in Crimea nor in training sessions.
"What kind of a parcel mama can you send me? What are you talking about? What happened?" he asked.
Britain's UN Ambassador Dame Barbara Woodward was among those at the UN to accuse the Kremlin of launching "indiscriminate attacks against men, women and children" and violating international humanitarian law.
She warned that Ukraine is on the brink of a "humanitarian catastrophe" at an emergency meeting of the Security Council.
"Missiles have rained down on Kharkiv, with cluster munitions hitting residential areas and injuring residents," she added.
"Disruption to supply chains has caused food shortages in Kramatorsk.
"The reckless bombing of an oil depot in Vasylkiv has unleashed toxic fumes in nearby communities.
"Violence in Kyiv has forced people to seek refuge underground, with many thousands, including the elderly and disabled, unable to evacuate."
The UK's permanent UN representative told the Security Council that "hundreds of civilians had been killed as a result of the Russian invasion" and seven million people had been displaced, with the figure "rising exponentially".
Mr Kyslytsya told the council that Kyiv was "sitting within Russian crosshairs right now" and that 352 people, including 16 children, had been killed as of Monday in the fighting.
He accused Moscow troops of attacking hospitals and ambulances in a determination to "kill civilians", adding "there is no debate. These are war crimes."
But Vasily Nebenzya, the Russian UN permanent representative, said his country's armed forces did "not have the goal of occupying Ukraine or harming the local population".
The chief prosecutor of the International Criminal Court said he plans to open an investigation "as rapidly as possible" into possible war crimes and crimes against humanity in Ukraine.
Ukrainian President Volodymyr Zelensky echoed Mr Kyslytsya's statements in a late-night address on Monday.
In a video posted to social media, the leader said that in five days Russian forces had launched 56 missile strikes and 113 cruise missiles in Ukraine.
He added: "Today, Russian forces brutally fired on Kharkiv from jet artillery. It was clearly a war crime.
"Kharkiv is a peaceful city, there are peaceful residential areas, no military facilities. Dozens of eyewitness accounts prove that this is not a single false volley, but deliberate destruction of people.
"The Russians knew where they were shooting."
He added: "There will definitely be an international tribunal for this crime, it's a violation of all conventions. No one in the world will forgive you for killing peaceful Ukrainian people."
It comes as Prime Minister Boris Johnson flies to Nato's border with Russia today, pledging that Vladimir Putin will "feel the consequences" for invading Ukraine.
Prime Minister Boris Johnson has said that UK troops will not fight Russia in Ukraine under any circumstances.
He also repeatedly rejected calls for a no-fly zone over Ukraine despite pleas to stop attacks from Russian jets.
Mr Johnson met counterparts in Poland and Estonia as he pushes for Western unity in punishing the Russian president for starting a conflict that has taken "hundreds" of lives in only five days.
Mr Kyslytsya yesterday compared Putin to Adolf Hitler as the UN held a rare emergency session of its General Assembly in New York.
He said the Kremlin's "spiritual mentors" were "from the Third Reich" and told delegates "this war wasn't provoked" but was "chosen by someone who is right now sitting in a bunker."
Secretary-General Antonio Gutteres told officials that the invasion was "totally unacceptable" and said "enough is enough."
