The 21-year-old, who goes by the nom de guerre Zhura, says he is ready if Russian President Vladimir Putin decides to send troops across the border.

"I cannot rule out the possibility of the start of full-scale onslaught," he says, wearing a green balaclava, helmet and bulletproof vest.

Western governments and independent researchers say Moscow has been moving arms and troops towards the Ukrainian border in recent weeks, with Washington saying it has "real concerns" over the troop build-up.

If Russia decides to take action, Zhura says he and his battle-hardened compatriots will fight back.

"We will give a worthy response to the enemy."

Speculation has been rife of new moves in the longstanding conflict between Russia and its ex-Soviet neighbour, though Moscow has denounced reports of an invasion plan as Western "hysteria".

President Volodymyr Zelensky said Friday that Ukraine was "entirely prepared for an escalation".

"We need to depend on ourselves, on our army. It is powerful," he told a press conference.

Zhura says the area of the frontline where he is deployed, near the village of Talakivka in the eastern Donetsk region, has been relatively calm in recent weeks.

"But it's only here," he said. "In the direction of another brigade, shelling is heard every day and it is quite strong."

Ukrainian forces are spread along a frontline that stretches along two separatist regions, Donetsk and Lugansk, that broke from Kiev's control after Russia annexed the peninsula of Crimea in 2014.

Though full-scale fighting has subsided, there are regular skirmishes along the front, with more than 60 Ukrainian soldiers killed this year.

The new buildup of forces follows a similar surge in the spring, when Russia gathered around 100,000 troops on Ukraine's borders.

- 'Calm before the storm' -

Moscow later announced a drawdown, and some experts at the time said the troop movements may have been posturing ahead of Putin's high-profile summit with US President Joe Biden in June.

Armed with a Kalashnikov rifle, another Ukrainian soldier takes part in a shooting exercise a few kilometres away from Zhura's position.

"It's pretty quiet right now, but it may be the calm before the storm," 21-year-old Anatoliy, from the western region of Chernivtsi, told AFP.

Anatoliy, who did not give his last name, said he believes there is a "very high" risk of all-out war with Russia.

"Everything depends on the Ukrainian authorities and the international community, and if they support Ukraine," he added.

Kiev's soldiers have accumulated combat experience since 2014 and received arms and hardware from Western allies, particularly the United States which has committed $2.5 billion in support of Ukraine's forces since the outbreak of the conflict.

The pro-Western government has received Javelin anti-tank missiles, ammunition, and patrol boats from the United States, and has also bought Bayraktar drones from Turkey.

Anatoliy hopes that despite the rising tensions a full-scale war between Ukraine and Russia will never happen.

But, he added, "it is also possible that it is only a matter of time."