A Kyiv court Monday ruled that a 21-year-old Russian soldier who killed a civilian was guilty of war crimes and handed him a life sentence, in the first verdict against Moscow's forces since their invasion.

"The court has found that (Vadim) Shishimarin is guilty and sentences him to life imprisonment," judge Sergiy Agafonov said.

Shishimarin, a Russian sergeant, admitted earlier in court to killing 62-year-old Oleksandr Shelipov in the first days of the Kremlin's offensive in north-east Ukraine.

He was also found guilty of premeditated murder.

"The murder was committed with direct intent," judge Agafonov said.

"Shishimarin violated the laws and customs of war."

The Russian soldier told the court last week that he shot Shelipov under pressure from another soldier as they tried to retreat and escape back into Russia in a stolen car on February 28th, the fourth day of Moscow's invasion.

Shishimarin apologised and asked Shelipov's widow for forgiveness.

The youthful serviceman looked on from the glass defence box as the verdict was read out in Ukrainian. An interpreter translated for him into Russian.

Shishimarin's lawyer Viktor Ovsyannikov said he will appeal the verdict.

"This is the most severe sentence and any level-headed person would challenge it," Ovsyannikov, said following the court session, adding, that: "I will ask for the cancellation of the court's verdict".

He said that "you can feel societal pressure" on the decision of the court. 

The Kremlin said before the sentencing Monday that while it was "concerned" over Shishimarin's fate, it was unable to provide on-the-ground assistance because there is no Russian diplomatic presence in Ukraine.

"That doesn't mean we won't try through other channels. The fate of every Russian citizen is of paramount importance to us," Kremlin spokesman Dmitry Peskov told reporters. 

The landmark ruling is expected to be followed by others, with Ukraine opening thousands of war crimes cases since Moscow's invasion.