Braced against the driving snow, fresh-faced Ukrainian conscripts stare out toward the border with Russia, waiting to repel an invasion from invisible-but-feared troops amassed on the other side.

The soldiers trudge around in cloying black mud in the eastern Ukrainian countryside, where army tents have sprung up on a chicken farm only a few kilometres from the boundary with Russia.

Officially they are conducting military exercises. But with a reported 40,000 Russian troops gathered along the border just weeks after annexing Crimea, these soldiers are not willing to give up their part of the country without a fight.

"Tension does exist but I can't say that we feel it strongly or are afraid," said Major Theodor Shevchenko, his cheeks wet from the snow whipped sideways by frigid gusts of wind.

"We are carrying out military exercises on our own soil and we don't feel any fear. Of course we understand that the integrity of our state depends on each of us, from soldiers up to officers and... we are ready in case of any aggression to defend our people."

He won't say how many men are deployed in the region but confirms there are military exercises happening at several points along the border, a mostly invisible frontier which cuts through swathes of winter-hardened grassland and forest that were largely unguarded until now.

A few armoured personnel carriers stand to one side and a tank dug into a trench is mostly covered by camouflage netting near a few army tents, a deliberate message to the Russians that rolling into other parts of eastern Ukraine would not be as simple as it was in Crimea.

- 'Armed robbery' -

Soldiers in the Black Sea peninsula, which was overrun by Russian troops in a matter of days after the fall of a pro-Moscow government in Kiev, faced a humiliating withdrawal after citizens voted in a March referendum to breakaway from Ukraine.

Ukraine's Prime Minister Arseniy Yatsenyuk hit out at Russia for carrying out "an armed robbery" and said the country would respond if it tried to annex the eastern regions, home to many Russian speakers as well as industries on which Moscow relies.

"I want to officially warn Russia: we will respond firmly, including through military means, against any attempt to seize Ukraine, to cross borders, or annex eastern or other regions by Russian troops," Yatsenyuk said.

Moscow has brushed off concerns about the massive military drills it is carrying out near the Ukrainian border. Reports it had withdrawn a battalion of about 500 soldiers have done little to ease what the European Union said Friday was a "very dangerous" situation.

"Currently, it is impossible to talk about any concrete picture of what is going on near our borders. Troops are being withdrawn at some stretches and regrouped somewhere else," Ukraine foreign ministry spokesman Yevhen Perebiynis said.

Far from the militant rhetoric of diplomats and NATO warnings that the Russians are poised to attack, the youthful troops' life at camp mostly consists of daily military routines.

- 'Ready for real fighting' -

Their temporary home is a tent crammed with bunk beds, lined up around a glowing wood stove and a heap of rifles and helmets.

Lieutenant Yevgen Skiba admits the early morning starts and training drills are "hard sometimes when it is cold".

"We don't see any enemy. For us this is a normal training exercise," he told AFP, but adding that he was "ready to participate in real fighting".

"Of course I will defend my country," he said.

Ukrainian officials are guarded on the size of the border deployment of an army that experts say is ill-equipped to stave off a Russian invasion after years of underinvestment.

Yuriy Georgiyevskiy, the deputy governor of the Kharkiv region, said only that the buildup of Russian troops "was significant enough for us to secure the defence of 270 kilometres (170 miles) of the state border.... The situation with the movement of Russian troops required an adequate response."

Security has also been stepped up at the main border crossing into the region, just 40 kilometres (25 miles) from Ukraine's second city Kharkiv, the one-time capital of Soviet Ukraine.

Sandbags and concrete blocks have been placed in front of the main checkpoints and a few kilometres away, tanks and armoured cars are stationed on the side of the road.

Dmytro Zhuravel, captain of the Ukrainian border guards, said defence of the frontier had gone into "enhanced mode," with staff doubled in recent weeks, checking every car thoroughly that passed through.

"Now in the checkpoint zone you can see special barriers intended to prevent forced crossings by any vehicles, both from Ukraine to Russia and from Russia to Ukraine," he told AFP.

He estimated that the number of people crossing the border had gone down by 20 percent compared to previous years.

However for Zhuravel, like many of those involved in the buildup of security along the border, the Russian threat remains largely invisible.

"We are not seeing any aggression from the Russian side.... We don't feel any threat," he said.