Vitali Klitschko, the Kyiv mayor and former heavyweight boxing champion, told Reuters Thursday that Ukrainians will continue to defend their homeland from Russian President Vladimir Putin and Russian invaders.

Klitschko was joined by brother Wladimir during a Skype interview with the outlet.

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Klitschkos

Vitali Klitschko, Kyiv Mayor and former heavyweight champion, right, and his brother Wladimir Klitschko, a Ukrainian former professional boxer look at a smartphone in City Hall in Kyiv, Ukraine, Feb. 27, 2022. (AP Photo/Efrem Lukatsky)

"Thousands, already tens of thousands, are killed in this war against Ukraine. And this number will unfortunately only grow," Vitali Klitschko said while speaking from a shelter he is working from in Kyiv.

"We are not going to surrender. We have nowhere to retreat."

Klitschko said only about half of Kyiv’s population remains, and authorities helped many orphans leave for Poland and Germany.

"The situation is tense, people are worried," Klitschko said. "The city needs peace and peaceful skies … Kyiv residents have been in their basements nonstop for a week now because of air raid alarm going off all the time."

Vitali Klitschko, Kyiv Mayor and former heavyweight champion gestures while speaking during his interview with the Associated Press in his office in the City Hall in Kyiv, Ukraine, Sunday, Feb. 27, 2022. A Ukrainian official says street fighting has broken out in Ukraine's second-largest city of Kharkiv. Russian troops also put increasing pressure on strategic ports in the country's south following a wave of attacks on airfields and fuel facilities elsewhere that appeared to mark a new phase of Russia's invasion

Vitali Klitschko, Kyiv Mayor and former heavyweight champion, gestures while speaking during his interview with the Associated Press in his office in City Hall in Kyiv, Ukraine, Feb. 27, 2022.  (AP Photo/Efrem Lukatsky)

Klitschko acknowledged that Russia was attempting to seize Kyiv, and he called on Russians to challenge Putin.

"You are an instrument at the hands of one man with one ambition – to resurrect the Soviet Union ... For ambitions of one man, we are paying a very high price," Klitschko added.

Wladimir Klitschko added that there is "no reason to attack Ukraine."

Wladimir Klitschko

Former Ukrainian boxer Wladimir Klitschko (right) greets a staff member after he registered as a volunteer during a visit to a recruitment center in Kyiv Feb. 2, 2022. (Genya Savilov/AFP via Getty Images)

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"It's complete madness ... What is happening is terror. It's happening in 2022. It's something one's mind doesn't comprehend," Wladimir said. "You don't know if you're going to make it through the day tomorrow. Those nights, hearing explosions all night long and shooting in the street, that does make you feel worried.

"We're really counting hours here. We have no time. We need to act now, we need to stop this madness. All the money that Russia is getting is being used to buy lethal weapons that are killing Ukrainians."