Kyiv mayor Vitaly Klitschko, a former world heavyweight champion boxer, pledged Wednesday to "defend" the city, if warnings of a potential invasion by Russian forces come true.  

"We have to be prepared for the worst case scenario. We hope it does not happen," Klitschko told journalists Wednesday. 

"But if aggressors come to our country we don't have any choice, we have to defend our city, our country, our future."

Klitschko along with his brother, fellow champion boxer Wladimir, visited a recruitment centre in central Kyiv for volunteers willing to take up arms if the city is attacked.

Wladimir Klitschko signed up for the territorial defence, insisting he wanted to "protect" his family, friends and relatives.

"There are two ways -- you can run or you can face the challenge," Wladimir said. 

"So my choice was to face the challenge."

Tensions have soared over Ukraine as some Western leaders warn that an attack from the more than 100,000 Russian troops on Ukraine's border could be imminent. The Kremlin denies it is planning an incursion.

Ukraine's President Volodymyr Zelensky has called to avoid stirring "panic" -- and life continues as normal in Kyiv with people going about their daily tasks. 

Vitaly Klitschko insisted that he hoped a flurry of diplomatic efforts could help avert any attack -- warning that "if not, we have to prepare to take weapons in our hands".

"We are not weak and any aggressor has to understand if they do any aggressive activities against Ukraine they have to pay a huge price," he said, thanking Ukraine's Western backers for weapon deliveries.

He said that Kyiv's administration was making preparations in case an attack came, including releasing details of potential bomb shelters, and working together with the armed forces on plans to defend the city of around 3 million. 

But he said the authorities still required help getting in place emergency communication systems in case phone networks and internet went down. 

"We have to be prepared for any scenario," he insisted.