A Ukrainian court on Monday sentenced a Russian soldier to a life in prison in the first trial for war crimes held in the country since it was invaded by Moscow, the Associated Press reported.

Vadim Shishimarin was accused of killing a 62-year-old Ukrainian civilian Oleksandr Shelipov by shooting him through an open car window in the northeastern Sumy region on February 28, four days after the invasion had begun.

On May 18, the 21-year-old soldier had pleaded guilty to murder.

At the trial on May 18, Ukrainian Prosecutor General Iryna Venediktova had said that the Russian soldiers had spotted the elderly man walking on the sidewalk and talking on his phone.

Venediktova added that Shyshimarin was asked to kill the man so he would not be able to report presence of Russian troops to Ukrainian military authorities.

“I was ordered to shoot,” Shyshimarin had said. “I shot one [round] at him. He falls. And we kept on going.”

On Monday, Shyshimarin, a member of a tank unit, apologised to the man’s widow in the court, Al Jazeera reported.

“I regret it,” he said. “I regret it very much. I did not refuse and I am ready to accept any measures imposed.”

Ukraine-appointed counsel Victor Ovsyanikov, appearing for Shyshimarin, said that his client was not prepared for the “violent military confrontation” when the Russian forces invaded.

Ovsyanikov said he will file an appeal against the court’s ruling.

“This is the most severe sentence and any level-headed person would challenge it,” he added. “I will ask for the cancellation of the court’s verdict.”

After Shyshimarin’s sentence was announced, Ukrainian civil liberties advocate Volodymyr Yavorskyy said that the judgement was harsh for one murder committed during a war, according to AP.

But Aarif Abraham, a United Kingdom-based human rights lawyer, said the trial was conducted in a fair manner by giving the soldier access to an attorney.

Just before the sentencing, Kremlin spokesperson Dmitry Peskov said that Russia was concerned about the soldier’s fate but it could not do anything due to a lack of on-ground assistance in Ukraine, Al Jazeera reported.

“That doesn’t mean we will not try through other channels,” Peskov had said. “The fate of every Russian citizen is of paramount importance to us.”

According to the United Nations’ human rights office, the conflict has led to the death of at least 3,930 persons.