STORY: Standing in the way of the Russian advance in eastern Ukraine: a Ukrainian battalion and a unit of foreign nationals who answered Kyiv's call for help.

They're about half a mile from Russian positions defending the captured eastern city of Izium.

"From the Ukrainian people," is the message on this mortar.

Artillery rains down most nights.

Denis Polishchuk's nom de guerre is "Canada" because he was born in Ukraine but lived in Vancouver.

“What am I going to tell my children - God willing, I have them someday - when they grow up, or my grandchildren when they ask me about these truly historical times we're living in? And I felt that the only dignified response would be that, yes, I was doing my part. I was fighting alongside with everyone else.”

Polishchuk is part of the Carpathian Sich battalion - one of several paramilitary nationalist groups that began as volunteers in 2014, when Russia annexed the Black Sea peninsula of Crimea and backed pro-Russian armed separatists in Ukraine's eastern Donbas region.

Since May, Kyiv says they've been reformed and integrated into the regular army.

Moscow brands such former paramilitary groups far-right extremists - and justified its invasion by saying it wants to "denazify" Ukraine. They strongly reject the charge.

The fighters recently captured a Russian tank almost intact.

They also contend with Russian drones – which they call "black clouds" – that direct artillery fire to their positions.

A field commander, who gave his name only as Dzvin for security reasons, says if Russian forces broke through here other units could be outflanked.

“We understand that if we, God forbid, surrender this line, if this horde can break through, we will let a huge number, thousands of our brothers who are holding the Donetsk and Luhansk lines, get executed. So it is extremely important. Our deterrence makes it impossible to encircle our troops.”

Conor is an ex army medic. He says images of wounded women, children and fighters without adequate medical help prompted him to leave Britain.

“So, I thought some of the knowledge that I've been trained in, bring it out here, and we've helped set up field hospitals ... It is getting a lot tougher out here the longer it goes on. It is definitely tiring. Sleep patterns are broken from shelling, and so they shelled at one, two and four o’clock in the morning yesterday, so that’s obviously breaking our sleep routine up, but you've got to stay positive.”

Two Britons and a Moroccan citizen captured fighting with the Ukrainian army were sentenced to death as mercenaries by a Russian-backed separatist court in June.

Polishchuk says the threat of capture scares him, but not enough to deter him.

"It’s not going to stop me, it's not going to change my decision. It's definitely something that you have to keep in mind and consider, but at the same time this is war. We all know the possible consequences of us being here and we’ve all made peace with that.”