The first Russian soldier on trial in Ukraine for war crimes during Moscow's invasion pleaded guilty today, facing possible life imprisonment in Kyiv.

Asked in court if he was guilty of the allegations, including war crimes and premeditated murder, 21-year-old sergeant Vadim Shishimarin responded “yes”.

He is accused of killing a 62-year-old civilian in northeast Ukraine in the first days of the Kremlin's offensive.

Shishimarin — from the Siberian region of Irkutsk — sat in the glass defendant's box in a Kyiv district court, wearing a blue and grey hoodie.

The youthful-looking soldier with a shaved head looked towards the ground as a prosecutor read out charges against him in Ukrainian.

An interpreter was translating for him into Russian.

He is accused of killing the civilian — allegedly on a bicycle — near the village of Chupakhivka in the eastern Sumy region on February 28.

Prosecutors say Shishimarin was commanding a unit in a tank division when his convoy came under attack.

He and four other soldiers stole a car, and as they travelled near Chupakhivka they encountered a 62-year-old man on a bicycle, they said.

According to prosecutors, Shishimarin was ordered to kill the civilian and used a Kalashnikov assault rifle to do so.

The Kremlin earlier said it was not informed about the case.

Russian forces stand accused of committing war crimes during a conflict that has left thousands dead and forced millions to flee their homes.

These include the summary killing of civilians in places like Bucha, a small town outside of Kyiv, where AFP reporters witnessed bodies abandoned in the streets by retreating Russian invaders.

The International Criminal Court said on Tuesday it was deploying its largest-ever field team to Ukraine, with 42 investigators, forensic experts and support staff being sent into the field to gather evidence of alleged crimes.

And the US State Department also announced it was creating a special unit to research, document and publicise Russian war crimes.

The Conflict Observatory will “capture, analyse, and make widely available evidence of Russia-perpetrated war crimes and other atrocities in Ukraine,” the department said.

'Hundreds of Ukrainians surrender'

Meanwhile, hundreds of Ukrainian soldiers who held off Russian fighters at the besieged Azovstal steelworks in Mariupol have surrendered, Moscow said, as Kyiv called for an immediate prisoner swap.

The strategic port city fell to Russian forces last month, but a relentless Ukrainian military unit held out in the maze of tunnels under the plant, hailed as heroes and celebrated for stalling Moscow's invasion.

On Tuesday, 265 of them were taken into Russian captivity, including 51 who were heavily wounded, the Russian defence ministry said.

The ministry, which published images showing soldiers on stretchers, said the injured were transported to a hospital in the eastern Donetsk region controlled by pro-Kremlin rebels.

The defence ministry in Kyiv said it was hoping for an “exchange procedure... to repatriate these Ukrainian heroes as quickly as possible”.

The government would do “everything necessary” to rescue the undisclosed number of personnel still holed up in the Soviet-era bunkers, the ministry said, but admitted there was no military option available.

The fate of the captured Ukrainians was unclear Tuesday, with Kremlin spokesman Dmitry Peskov refusing to say whether they would be treated as criminals or prisoners of war.

President Vladimir Putin “guaranteed that they would be treated according to the relevant international laws,” Peskov said.

Trust between the two sides is in short supply, with Kyiv saying negotiations on ending the three-month conflict were on hold, blaming Moscow for a refusal to compromise.