The first Russian soldier on trial in Ukraine for war crimes during Moscow's invasion pleaded guilty on Wednesday, 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.

"We still have no information. And the ability to provide assistance due to the lack of our diplomatic mission there is also very limited," Kremlin spokesman Dmitry Peskov said.

Shishimarin's lawyer, Viktor Ovsiannikov, told AFP earlier that the Russian soldier "understands what he is being accused of", without revealing the case for the defence. 

Ukrainian authorities say he is cooperating with investigators and admits killing the civilian. 

In early May, Ukrainian authorities announced the soldier's arrest without giving details, while publishing a video in which Shishimarin said he had gone to fight in Ukraine to "support his mother financially."

He explained his actions saying: "I was ordered to shoot, I shot him once. He fell and we continued our journey."           

Two other Russian servicemen are due to go on trial in Ukraine on Thursday for firing rockets at civilian infrastructure in the north-eastern Kharkiv region.