The Ukrainian military has launched a new initiative to entice fighting-age men living abroad to enlist, as Kyiv desperately tries to replenish its depleted forces on the front lines.

The establishment of the so-called Ukrainian Legion comes after more than two years of fighting, which has seen heavy casualties drain the military of manpower needed to battle Russia's advancing forces.

The creation of the new unit was part of a wider security agreement signed by Ukrainian President Volodymyr Zelensky and Polish Prime Minister Donald Tusk earlier this week.

"We call on all Ukrainians in Europe to join the Ukrainian Legion. Your contribution is invaluable in our struggle for freedom and independence," Defence Minister Rustem Umerov announced on social media late Tuesday.

The initiative aims to recruit volunteers from the pool of Ukrainian men living in Europe, especially Poland and Germany, which are home to the largest numbers of Ukrainian refugees.

An estimated 300,000 Ukrainian men of fighting age are believed to be living in Poland alone.

Hundreds of thousands of Ukrainian men are believed to have been residing in Europe before the war, while an untold number of others fled after the Russian invasion -- some legally and others through illicit means.

The passage of a new law in May that lowered the minimum age for mobilisation to 25 from 27 has also prompted many others to avoid potential conscription.

Umerov said members of the new unit would receive training in Poland and be provided the best equipment.

Poland has appeared cautious over the initiative.

"At this stage, it is too early to talk about details. The issue is under discussion between the two countries' defence ministries," the press service for Poland's defence ministry told AFP on Wednesday.

Following the announcement, Ukraine's popular Third Assault Brigade said its representatives would be touring European countries soon to provide more information about their unit along with "the most epic stories from the front line".