We approach Ukrainian army positions through an icy fog that portends to the inevitable snowstorms that will soon envelop the east of the country.

The army unit we are with is dug in near Novoluhanske, an unremarkable mining town in Ukraine's once active industrial heartland. It's now all but closed down by the war that's brought chaos to this region since a Russian-backed separatist uprising in 2014.

Now this almost forgotten conflict is back at the top of the diplomatic agenda after Vladimir Putin decided to send tens of thousands of troops and equipment to Russia's western border with its once close neighbour.

What is happening and will tensions lead to war?

What open-source evidence tells us about Russia's military activity

Russia has balked at what it sees as NATO advances towards its western front for decades. President Putin is now issuing demands for a withdrawal, coupled with dire warnings of possible further military intervention.

Western allies are putting on a united front in response to Russia's tactics, writes Sky's Deborah Haynes, but whether that deters the Russian president remains to be seen.

Russia has already annexed Crimea and gave explicit support to the separatists grouped mainly around the city of Donetsk and the Donbas region.

As we walked towards the start of the frontline proper, the unit's commanding officer stopped and drew an arc in front of us.

"The separatists are 500 meters in every direction," First Lieutenant Anatolii Semenenko told me.

"We are here to stop them coming this way," he said, gesturing me towards a group of soldiers standing next to a small hill. "We built everything here in four months. Come see where some of my men sleep."

We walked down some muddy steps beneath the hill and entered a small warm room dug out of the earth. A litter of tiny puppies and their mother are in a wire cage in one corner, the rest of the room has a couple of bunk beds.

This is the frontline, home to four of the soldiers that guard this sector.

The soldiers told us they keep the dogs with them because when they sense incoming artillery, they run underground. The dogs can sense danger long before the soldiers can hear it - it's a useful warning system of sorts.

Like being transported back to WWI

From this subterranean billet, we were guided towards the most forward position through a myriad of trenches.

I felt like I had been transported back over a hundred years to the First World War.

It seems remarkable that after all this time, with all the military developments, and the invention of weapons systems that can kill by the use of a joystick thousands of miles away, the Ukrainian army and the Russian-backed separatists are fighting in WWI-style trenches, at points less than 100 meters apart.

The trenches snake along a 250-mile demarcation line, with both sides well dug in.

The narrow walkways and tunnels combined with the near silence all add to a dreadful sense of foreboding.

There are constant reminders that this is a war.

"Beware Sniper" signs mark specific sections of the trench. This is the most common way Ukrainian soldiers are killed. We are told to keep our heads down. I scuttle between these most exposed sections.

With heat-seeking cameras, the soldiers attempt to monitor the troops and equipment on the separatist side - they are trying to detect any discernible increase in numbers and sophistication.

Their frontline is a row of trees we can see - it's barely a hundred metres away.

There's been a ceasefire here since 2015, but it's constantly violated, both sides accusing the other.

And like the Christmases of the Great War, the troops on both sides dug in to their trenches are holding out for the new year - holding out and hoping they'll make it.

First Lieutenant Semenenko and his men are worried about Russia's threats of an invasion but think Putin might be bluffing.

'We know who the enemy is'

Whichever way, they're ready and believe their army is more prepared now than they were in the past.

"We now know for sure who the enemy is, because in 2014 when the war started, it was difficult for people in Ukraine to believe that our neighbour, who we considered brothers, would attack us. So mentally we have changed, mentally we have moved away from the Soviet Union," he explained to me.

He believes the Ukrainian army can withstand Russia's advances.

"Our soldiers have learned from all these years of war, and I think we will push back. We are on the defensive, so we have the advantage."

Looking out from the trenches and into the misty landscape, it feels like nobody is here. But the soldiers are here, their enemy is nearby, and just a few miles away are towns and villages.

These frontline towns in the industrial heartlands of Eastern Ukraine are tough-looking reminders of the Soviet past.

Many are now deserted. There are, however, many people too poor to leave.

One of them is Danya Kharchenko, a remarkable boy who lives in the half-abandoned town of Svitlodarsk with his mother and sister.

Every day, the 13-year-old gets on his bike to go fishing at the reservoir that cools a nearby power station. His uncle taught him how to build a fishing net before he left the town - his uncle, like so many, left because of the war.

Danya uses the net to catch fish, and what he catches he sells to local markets. Though his family has very little, he uses the money to feed the town's homeless dogs left behind by their owners who have evacuated.

He says people tease him and ask him why he uses the money for the dogs instead of himself, but feeding the dogs helps him feel better about the war all around him. It makes him feel like he's doing something to help.

The fighting has gone on for nearly half of Danya's life, in many ways, it's just become part of his life now.

"When they shoot, you hide under a hillock if you're in open terrain. If you are in town, you should run over to a [shelter] entrance or a basement if it is open", he explained matter-of-factly.

"It was scary at first, but we are used to the machine guns now. It's not as scary as it first was."

There are thousands of children like Danya all along the demarcation line here in Eastern Ukraine, caught up in a conflict they can't escape from.

He and those like him are still learning to live with a war that hasn't stopped for eight Christmases in a row.