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The IBF, WBO, and WBA champion has put his career on hold and returned to his home country from London in order to help the war effort.
According to TSN, the 35-year-old fighter has enlisted with the Kyiv Territorial Defence.
Usyk confirmed his decision to return to return to Ukraine in an Instagram Live video.
He said: "Dear Ukraine and dear Ukrainians, some wrote about me running away.
“Not true, I had a work trip and now I’m back. I’m home,
"Friends, we need to unite together and overcome this, because we are faced with exceptional difficulties.
"I’m very overcome with emotions, and I’m very concerned for my country and for our people. Friends we must stop this war and we must do it together."
In another video beforehand, he had spoken directly to the Russian people, urging them not to wage war.
Usyk – who took the heavyweight belts of Anthony Joshua with a masterclass last year – said: “Good morning to everybody,
“My name is Oleksandr Usyk."
“I’d like to speak to the people of Russia.
“If we consider ourselves as brothers, orthodox ones. Do not let your children to set out to our country, do not fight with us.
Addressing Russia’s President Vladimir Putin directly, he added: “Also I’m addressing this to the President Vladimir Putin.
“You can stop this war.
“Please just sit down and negotiate it with us without claims.
“Our kids, wives, grannies are hiding in the basements… We are here in our own country, we cannot do it other way – we are defending… Stop it! Stop this war.”
Usyk isn’t the only professional fighter to have decided to take up arms against Russia.
Former heavyweight world champions Wladimir and Vitali Klitschko – Vitali is the mayor of Kyiv – have also agreed to fight.
Three-weight champion Vasily Lomachenko has also reportedly joined a ‘territorial defence battalion’ in his hometown of Bilhorod-Dnistrovskyi, near to the Ukrainian border with Moldova.
From the world of MMA, the undefeated Bellator welterweight champion Yaroslav Amosov has also said that he will head back to Ukraine to take up the fight against the Russian invasion.
Usyk was due to take part in a rematch with Anthony Joshua later in the year, although nothing has yet been announced, and it is not known whether that will still happen given recent events.
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