Olga Salo arrived for her first tactical military training exercise in a frosty pine forest outside Ukraine’s capital Kyiv wearing a pink ski jacket and dark blue jeans.

On a frigid Saturday in December, when much of the world was celebrating Christmas, the 39-year-old museum guide lined up in troop formation alongside hundreds of other civilians who had volunteered to defend Ukraine’s home front in the event of a full-blown Russian invasion.

Prospective and new recruits were issued with wooden replica weapons. More experienced recruits and reservists, many clad in camouflage gear, carried automatic rifles.

“It’s necessary to be prepared and react properly to the worst-case scenario, ” Salo said. “If we will be ready, maybe it won’t happen. I think the enemy will not attack if he knows that he will be resisted here not only by the regular army but by the population. ”

The rush of volunteers comes as tensions escalate amid reports that Russia has amassed 100,000 troops along its border with Ukraine, sparking fears that Moscow is preparing to invade. As part of intense diplomatic efforts to ease the crisis, US president Joe Biden and Vladimir Putin, his Russian counterpart, spoke by phone on Thursday, ahead of further negotiations in January between Washington, Moscow and Nato powers.

Although Putin has previously denied any plan to invade, he said last week he was prepared to use “appropriate military-technical measures” and “react harshly to hostile steps” should Ukraine and its western backers ignore Moscow’s “red lines”. These include a freeze on further western military assistance to Ukraine, rejection of Kyiv’s bid to join Nato and a pullback of the military alliance’s forces from eastern Europe.