A Russian soldier who pleaded guilty in the killing of an unarmed 62-year-old Ukrainian man has been sentenced to life in prison as part of what was the first war crimes trial of Russia's ongoing invasion of Ukraine.

Vadim Shishimarin appeared in court during his sentencing on Monday (May 23) and was told that he "committed a criminal offense" in adherence with international humanitarian law during the incident in late February.

"[Shishimarin] saw a civilian on the pavement, Oleksandr Shelipov," the court said via CNN. "Shishimarin knowing that Shelipov is a civilian and is unarmed and does not pose any threat to him -- fired several shots at Shelipov from his AK-gun."

The court said Shishimarin shot Shelipov in the head, which resulted in the 62-year-old's skull being crushed.

Russia presidential spokesperson Dmitry Peskov said the Kremlin was "concerned" over the sentencing while addressing reporters during a regularly scheduled conference call.

"We do not have many opportunities to protect his interests on the ground, as foreign institutions actually have no activity [in Kyiv]. But this does not mean that we will not consider the possibility of making attempts through other channels," Peskov said, though not specifying which channels he was speaking about, and adding that Russia considered the charges "unacceptable," "outrageous" and "staged," via CNN.

Shishimarin pleaded guilty to the charges on May 18, BBC.com reported.

"Do you accept your guilt?" the judge asked Shishimarin. "Yes," Shishimarin replied.

The victim's widow was present in the courtroom during Shishimarin's plea and sentencing.

Prosecutors said Shishimarin was leading tank division unit that came under attack before he and four ally soldiers stole a car and travelled near Chupakhivka, which is where he encountered the victim, who was riding a bicycle at the time.

Prosecutors said the 21-year-old was ordered to kill the 62-year-old and used a Kalashnikov assault rifle, BBC.com reports.

Putin announced his country would conduct military operations in Eastern Ukraine during an NBC News translation of a speech addressing the Russian population in Moscow on February 24.

The announcement appeared to serve as the final action ahead of an attack by Putin and the Russian military, which the U.S. and European allies to the neighboring Ukraine have attempted to prevent from taking place through diplomatic discussions.

A Ukraine interior minister confirmed to NBC News via text message that "cruise and ballistic missile strikes" were already underway shortly after Putin's announcement.

NBC News correspondent Erin McLaughlin said explosions could be heard from her live shot in Kyiv, Ukraine's capital city, at 6:00 a.m. local time.

More than 1,000 protesters were reportedly arrested during anti-war protests throughout Russia amid President Putin's announcement to conduct military operations and ensuing attack on Ukraine, BNO News reported.