Wladimir Klitschko joins Ukraine reservists

Both Wladimir and his brother Vitali Klitschko, Mayor of Kyiv and also a former heavyweight boxing champion, were present during the opening of the nation's first Territorial Defense Forces recruitment center in Kyiv.

Wladimir Klitschko said he felt obliged to defend the country.

"When seeing my country is geopolitically struggling, so I am right here. I can't stand still - and I won't," he told reporters.

A diplomatic solution to the conflict with Russia is preferred, according to his brother.

"If not, we have to prepare to take weapons in our hands, and defend the country," he said.

Russia has massed more than 100,000 troops on the Ukrainian border and Western countries say they fear Putin may be planning to invade.

Russia denies this but has said it could take unspecified military action unless its security demands are met. Western countries say any invasion would bring sanctions on Moscow.