Mr Klitschko, who has been Mayor of Kiev in 2014, insists: “I once swore to defend the country and am now ready to fight for my motherland.” He has asked Germany for help in the ongoing Russia-Ukraine dispute, noting that the situation is “very, very serious” and will only get worse if Russia is not forced to back down.

       Writing in Bild, Mr Klitschko said the whole of Ukraine is preparing for President Putin to send Russian troops into the country, “as they did in 2014”.

       In his own city, he highlighted efforts to increase the number of military reserves as well as to ramp up civil defences.

       But he added that Ukraine cannot be expected to act alone.

       Mr Klitschko wrote: “What Germany should understand: The situation is very, very serious!”

       He added: “In Ukraine we urgently need: international support and military aid.

       “It cannot be that Germany did not approve the decision of the Nato on the delivery of defence weapons to Ukraine! The new Federal Government must understand that aid has never been as important for our country.

       “Ukraine is in the centre of Europe! On the border with several EU countries.”

       Foreign ministers for G7 countries warned Russia over the weekend that there would be “massive consequences” should it invade Ukraine.

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       In a statement, they said: “Any use of force to change borders is strictly prohibited under international law.

       “Russia should be in no doubt that further military aggression against Ukraine would have massive consequences and severe cost in response.”

       Mr Klitschko stressed that the west – Europe in particular – should do more than issue statements.

       He wrote: “We are a European country that needs European support more than ever!”

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       NBC News reports that the eastern edge of Ukraine contains scenes “reminiscent of World War I”.

       Russian tanks, along with tens of thousands of troops, have made their way to the Ukraine border.

       The Kremlin has insisted it has no plans to openly enter the conflict in Ukraine, but will defend its red lines on the country.

       Primary among these is that Ukraine must not join Nato.

       Political analyst Tatiana Stanovaya told the BBC: “I think for Putin it's really important. He thinks the West has begun giving Ukraine's elite hope about joining Nato.

       ”The training, the weapons and so on are like a red rag to a bull for Putin and he thinks if he doesn't act today, then tomorrow there will be Nato bases in Ukraine. He needs to put a stop to that."

       Additional reporting by Monika Pallenberg.