A machine guns rattles as pale and exhausted teams of Estonian weekend warriors struggle to climb a final obstacle: the wall of Narva Castle facing their country's powerful neighbour Russia.

The bullets fired on the snowy banks of the Narva river separating Estonia from Russia are blanks, but the steely determination of volunteers participating in Utria Assault, the NATO member's biggest annual military competition, is palpable.

Ruth Maadla, a waitress who spends her weekends as a paramilitary volunteer, told AFP she would give her all to help defend the small Baltic nation of 1.3 million people "if anything ever happened".

Sporting white winter camouflage gear, a headlamp and a huge backpack, the 29-year-old who has just finished a brutal 90-kilometre (56-mile) marching race in bone-chilling subzero temperatures is in high-spirits, despite being caked in mud and nursing painful blisters on her heels.

Like other east Europeans, Estonians were deeply disturbed by Russia's 2014 annexation of Crimea and its subsequent support for separatists in eastern Ukraine.

US President-elect Donald Trump then raised more concerns with his campaign threat to think twice about defending NATO's eastern allies.

These factors coupled with Kremlin sabre rattling in the Baltic region -- especially in its heavily militarised Kaliningrad exclave -- have triggered a paramilitary revival in eastern European states that were under Moscow's thumb during the Soviet era.

Part of the USSR until 1991, Estonia has seen its Kaitseliit volunteer paramilitary force expand by 10 percent over the last two years.

- 'Citizens with strong will' -

With 16,000 members -- up to 25,600 including units for women and children -- the organisation is seen as a crucial extension of the EU member's modest military force comprising 6,500 peacetime personnel, half of them conscripts.

While some paramilitary volunteers play war games to hone skills like shooting or orienteering, others prefer more peaceful duty like wielding knitting needles to make socks for war victims in eastern Ukraine.

The Kaitseliit force has even attracted some volunteers who are ethnic Russian, part of Estonia's largest minority accounting for about a quarter of its population.

Kaitseliit commander, Brigadier General Meelis Kiili, describes the force he leads as "a very important element in deterrence" when facing Russia.

The role of "ordinary citizens with a strong will to defend" must not be underestimated, Kiili told AFP, as he congratulated a troupe of volunteers exhausted after the two sleepless nights they spent marching through snowy forests in the race.

Many are former military conscripts, but more and more ordinary Estonians and women, like Maadla or Sille Laks, are joining.

A 30-year-old cyber security expert from Tallinn, Laks told AFP that she has spent around 400 hours in Kaitseliit basic training over three months.

"It's about doing something for my country," said the athletic public servant as she braved the freezing cold before dawn to supervise one of the checkpoints in the competition.

While NATO's collective defence clause is the bottom-line guarantee of Estonia's security, analysts acknowledge that paramilitaries do have a role to play.

"In the worst case scenario, Russia could advance very swiftly to take all of Estonia, but with its own resistance, Estonia could buy more time" for help to arrive, said Kristi Raik, a senior Baltic security researcher at the Finnish Institute of International Affairs, stressing that any such attack was unlikely at the moment.

- Under NATO's wing -

Moscow upped the ante in the Baltic region late last year by deploying nuclear-capable Iskander missiles into its Kaliningrad outpost bordering NATO member Lithuania and Poland and sending two ships capable of launching warheads to the Baltic Sea.

The move came on the heels of NATO's decision to deploy four multinational battalions to eastern Europe, including a 1,100-strong rotational unit that will be stationed as of April at the Tapa military base, an hour's drive from the Estonian capital Tallinn.

Over the next few months, the United States will also deploy part of a 3,500-troop armoured brigade to Estonia and Baltic neighbours Latvia and Lithuania.

They have all eyed Trump's pro-Moscow rhetoric with mounting unease.

Ordered by the outgoing Obama administration to reinforce NATO's vulnerable eastern flank, the US brigade arrived in Poland last week as part of one of the largest deployment of US forces in Europe since the Cold War, an operation that Moscow angrily branded a security threat.

While the advent of a Trump presidency adds an element of uncertainty to future US commitment to defend vulnerable eastern European allies, Estonia's paramilitary chief remains confident about NATO's resolve.

"It's not only Trump we are talking about, NATO has 28 members," Kiili told AFP.