More than 4,000 recordings of Russian soldiers making calls from Kyiv were obtained by The New York Times.
One Russian soldier told his girlfriend that he received orders to "kill everyone we see."
UN investigators previously accused Russia of committing war crimes in Ukraine.
One Russian soldier in Kyiv recounted in a phone call how his commander ordered troops to kill Ukrainian civilians to conceal their locations, which is a war crime under international law.
The call was just one of many unauthorized phone calls made in March by Russian soldiers, which were intercepted from the Bucha area - a suburb of Kyiv where civilians were massacred - by Ukrainian law enforcement agencies. The New York Times obtained, verified, and translated the recordings.
The calls reveal the bleak reality Russian soldiers faced in the early weeks of the war in Ukraine, which began in late February. The Times report documented soldiers reporting back to their mothers, friends, and girlfriends on some of the atrocities they witnessed - and participated in.
In one call to his girlfriend, a Russian soldier identified only by his first name, Sergey, said his commander had given an order to take Ukrainian civilians to the forest to execute them.
"They told us that, where we're going, there's a lot of civilians walking around. And they gave us the order to kill everyone we see," Sergey said, according to The Times.
His girlfriend responded: "Why the fuck?"
"Because they might give away our positions. ... That's what we're fucking going to do, it seems," Sergey replied, per the report. "Kill any civilian that walks by and drag them into the forest. ... I've already become a murderer. that's why I don't want to kill any more people, especially ones I will have to look in the eyes."
Killing civilians is a war crime under the Rome Statute of the International Criminal Court.
According to the statute, "intentionally directing attacks against the civilian population as such or against individual civilians not taking direct part in hostilities" would be considered one of the "serious violations of the laws" in international armed conflict.
The United Nations previously accused Russia of committing a long list of war crimes in Ukraine.
Some atrocities listed include sexual violence, torture, and "indiscriminate attacks with explosive weapons," according to Erik Møse, who led the UN's investigation into human rights violations and crimes committed in Ukraine.
In another recording between Sergey and his girlfriend, the Russian soldier said a captain ordered the execution of three men, in part because their unit didn't have enough food to feed themselves.
"We detained them, undressed them, and checked all their clothes. Then a decision had to be made whether to let them go," Sergey said. "If we let them go, they could give away our position. ... So it was decided to shoot them in the forest."
When his girlfriend asked why they weren't taken as prisoners, he replied: "We would have had to feed them, and we don't have enough food ourselves, you see."
The UN wasn't the only international entity to suspect Russia of committing atrocities in Ukraine. Russian forces, who have repeatedly targeted civilians since the war began, have been widely accused of committing war crimes in Ukraine - including by world leaders and top human rights groups.
President Joe Biden has gone as far to accuse Russian President Vladimir Putin of committing "genocide" in Ukraine. Biden said in April that Putin was vying to "wipe out the idea of even being a Ukrainian."