2 of the world's best boxers have joined the fight for Ukraine against Russia's invasion

• Two of the best boxers in the world flew to Ukraine to help the war effort against Russia invaders.
• Oleksandr Usyk and Vasiliy Lomachenko are not alone in taking up arms.

Vasiliy Lomachenko and Oleksandr Usyk, two of the world's best active boxers, have joined Ukraine in the fight against Russia's invasion.

Russia sent troops into the former Soviet nation Thursday and shelled several cities including Kyiv, the capital, and Kharkiv, which is 25 miles from the border with Russia.

Kharkiv's resistance began Friday, and has become a "key battleground" in Ukraine, Washington Post reported Monday.

The country's key boxing figures have joined the war effort.

Lomachanko, a two-time Olympic gold medalist and three-weight world champion boxer, joined a territorial defense battalion in Ukraine.

Usyk, another Olympic hero who unified championship belts at cruiserweight and heavyweight, recently joined the Kyiv Territorial Defense and can be seen holding an assault rifle in one photograph.

"Good morning, my name is Usky Oleksandr," he said in one video this week. "I want to appeal to the Russian people. If you consider us brotherly people, do not let your military, your children, go to our country. Stop attacking us!

"I also appeal to [Russia] President [Vladimir] Putin. Stop this war! Don't give us ultimatums and declare your conditions, just sit down at the negotiating table. We need to talk and stop everything.

"Our children, our mothers, grandmothers, and ordinary people, are hiding in basements," said Usyk.

"We are on our own land, we act as we have to, we are protecting ourselves. Stop it. Stop this war."

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Both Lomachenko and Usyk are due to contest separate fights in the coming months, but it is unclear if the war effort will postpone those plans in boxing.

Lomachenko had been linked to a lightweight title fight against George Kambosos Jr. in Australia, and Usyk remains contractually bound to a rematch clause to defend his newly-won heavyweight belts against Anthony Joshua.

They are not the only boxers involved as legendary heavyweight Vitali Klitschko is a former champion who is now mayor of Kyiv.

Klitschko's brother Wladimir, also a former champion, took up arms against Russia, as Insider previously reported.

Insider was also ringside in Las Vegas for Saturday's Premier Boxing Champions event on Showtime Sports which featured a Ukrainian boxer, Viktor Postol, who lost to American fighter Gary Antuanne Russell.

Postol said earlier in the week that he would fly to Ukraine after his match and protect his family.