A Russian soldier has pleaded guilty in the first war crimes trial related to the ongoing invasion of Ukraine.

Vadim Shishimarin, 21, admitted to fatally shooting a 62-year-old man just days after Russian President Vladimir Putin launched his invasion on the neighboring Eastern European country in February, BBC.com reports.

"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 seen wiping tears from her eyes as he entered the court, BBC.com reports.

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.