"Glory to Ukraine," said Igor Rusakevych as he signed up as a volunteer Tuesday with the Ukrainian Legion that Kyiv has created for its nationals living abroad.

Ukraine has been struggling to bolster its ranks after nearly three years of Russia's invasion and the legion is its latest push to convince men of combat age to enlist.

Its recruitment office, opened last month in the Polish city of Lublin, around 100 kilometres (60 miles) from the Ukrainian border, has received about 700 applications so far.

"I had to finish some things. I have a family here, a good job. I had to gather my strength," Rusakevych said, explaining why he had not joined the army earlier.

The 37-year-old construction manager has lived in Poland for more than 30 years, has a teenage son and a Polish wife.

"I feel like a Pole, but I am a Ukrainian at heart," he said.

"I watch the news every day. I see that we are losing."

Russian forces are advancing steadily in eastern Ukraine and have intensified their assault on the southern front, as well as keeping up barrages of drone and missile attacks on cities across Ukraine.

- 'To redeem myself' -

The Ukrainian defence ministry said the first group of volunteers who signed contracts in Lublin had immediately gone for basic military training lasting about a month.

As the legion was established as part of a wider security agreement between Warsaw and Kyiv, Poland hosts initial training before the new soldiers are sent to continue at NATO bases in Europe.

Another volunteer, 20-year-old Dmytro Zdorik, who previously lived in Lithuania, said he had felt "drawn to" the army since childhood.

He said he found out about the legion from his grandfather, himself a former military man.

"He sent me a video. And I thought, here it is -- a chance to quickly join the army," Zdorik said.

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

Through its online advertising, Ukraine aims to reach out to its nationals residing across Europe.

Yuriy Kulius, a former prisoner, said he had spent two years in Germany before deciding to enlist in the legion -- a move that he had been mulling "for a long time".

"I just want to redeem myself... I am tired of the stigma of a criminal record," the 45-year-old said.

"The main thing is... to be useful," said Kulius.

He said he had shared the news about joining the military with his mother, who still lives in Ukraine.

"She said I was an idiot," the man recalled, before adding with a chuckle: "Well, I don't search for an easy way in my life."