Ukrainian teenager Mykhailo Anopa had been suffering sleepless nights worrying about a full-on attack from Russia. Then he decided to do something about it.

The 15-year-old joined other boys from disadvantaged families digging trenches for soldiers serving on his country's eastern front and facing off with Russian-backed separatists.

"When our father, the pastor, said there might be shelling, that Putin might attack, I started having nightmares, because that's what I would think about before bed," he said.

Facing fears of a Russian invasion, Ukrainian teenagers from broken homes have decided to respond by digging trenches for their troops
Facing fears of a Russian invasion, Ukrainian teenagers from broken homes have decided to respond by digging trenches for their troops AFP / Aleksey Filippov

"We dug trenches to help Ukrainian soldiers. Now, we are reinforcing them. This has become our responsibility," he said with pride.

Ukraine has been trapped in a festering conflict across parts of its mostly Russian-speaking industrial east ever since a pro-EU revolt deposed a Moscow-backed president in 2014.

The war has claimed more than 14,000 lives and forced 1.5 million people from their homes.

It has also drained Ukraine's economy and remained a constant drag on the former Soviet republic's aspirations to enter the Western fold.

'The children laugh and play a lot,' says pastor Gennadiy Mokhnenko. 'But buried beneath that are huge fears'
'The children laugh and play a lot,' says pastor Gennadiy Mokhnenko. 'But buried beneath that are huge fears' AFP / Aleksey Filippov

Now Ukrainians fear Putin's confrontation with the West over NATO's expansion could lead to an all-out invasion by the more than 100,000 Russian troops poised just across the border.

Anopa belongs to a centre for children from broken homes overseen by Pentecostal pastor Gennadiy Mokhnenko.

The 53-year-old has just returned to the Ukrainian front after a trip to the United States and is busy instructing his group of 40 or so boys about discipline and proper trench-digging techniques.

Ukraine's outnumbered and outgunned forces have seen a sharp rise in Russian naval activity in the strategic Sea of Azov region
Ukraine's outnumbered and outgunned forces have seen a sharp rise in Russian naval activity in the strategic Sea of Azov region AFP / Aleksey Filippov

"Today, we are going to reinforce basements," the pastor says, after leading a group prayer.

"We will buy some more gas canisters, develop an evacuation plan. Each one of us has to prepare a small bag with clothes and documents," he tells the group.

"In the evening, I will check how ready you are."

Some of the teenagers say they cannot sleep at night because of constant worries about an even more serious war breaking out
Some of the teenagers say they cannot sleep at night because of constant worries about an even more serious war breaking out AFP / Aleksey Filippov

Pro-Western sentiment runs high in Mokhnenko's class. The pastor's office is adorned with Ukrainian and US flags whose poles are supported by empty artillery shells.

Mokhnenko himself is dressed in house slippers and a camouflage Western-style uniform.

"The situation is very serious," he tells the boys gravely. "But we will be ready."

The trenches around the eastern village of Chervone, on the Sea of Azov coast, were first dug during another but much smaller military escalation in November 2018.

Then, a Russian warship shot at and intercepted a Ukrainian ship that had tried to pass from Ukraine's Black Sea port of Odessa to the Sea of Azov.

But the Kremlin's annexation of Ukraine's Crimea region in the weeks following the 2014 revolution makes such trips treacherous.

Crimea is a peninsula that controls a strait connecting the two seas, and the strait is patrolled by Russian gunboats.

The resulting clash in 2018 saw shells fall near Chervone and prompted Ukrainian soldiers to start preparing for a bigger war.

On a clear day, the boys in Mokhnenko's class can now see more Russian warships moving across the Sea of Azov.

A week of much bigger naval manoeuvres involving more than a dozen Russian warships and nearly 20 other boats are setting nerves jangling in the Black Sea.

Pastor Mokhnenko says boys such as Anopa are more cheerful when they feel like they are doing something constructive such as digging trenches.

"But buried beneath that are huge fears. Years of their lives were spent watching the backs of soldiers. And out of their windows, all they see is the front."