At least 21 people were killed during a Russian missile attack near the Ukrainian port of Odesa on Friday (July 1), authorities confirmed to the Associated Press.

Videos of the attack shared publicly showed ruins of buildings in the town of Serhiivka, which is located about 31 miles from the port of Odesa and Ukrainian President Volodymyr Zelenskyy's office confirmed three Kh-22 missiles were fired by warplanes and struck an apartment building and campsite in the area.

Ukrainian officials believe the attack was retaliation by Moscow after Russian troops were forced from Snake Island -- which was seized by Russian troops early in their ongoing invasion of Ukraine -- on Thursday (June 30), despite the Kremlin portraying their departure as a "goodwill gesture" to unblock exports of grain amid a global shortage.

Russian President Vladimir 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.