TALLINN, Estonia (AFP) -- Russia's arrest of an Estonian policeman has sparked fears that Moscow is using the incident to test NATO's resolve to defend its small Baltic allies as the crisis in eastern Ukraine rumbles on.

Moscow detained officer Eston Kohver on Saturday in what Russia claims is a spying probe. Prosecutors say he was arrested in possession of a pistol, ammunition, 5,000 euros ($6,500) and "special equipment to carry out covert recordings".

But Tallinn alleges Russia kidnapped Kohver from Estonia at gunpoint as he was investigating cross-border crime.

A source close to the probe in Moscow quoted on Monday by Russia's Interfax news agency said Kohver had denied being a spy or smuggling a firearm into the country.

Many in Estonia believe the timing of Kohver's arrest is deliberate, coming just two days after US President Barack Obama visited Tallinn to trumpet Baltic security following Russia's role in the Ukraine crisis.

"The operation was obviously carefully planned and prepared," Kaarel Kaas, an analyst with Estonia's International Centre for Defence Studie, told AFP.

"The kidnapping on Estonian soil was a conscious choice to demonstrate that the FSB (Russian intelligence service) can operate in spite of all obstacles," he said.

"It's hard to tell if it was done to show NATO, Estonia or the KAPO (Estonian intelligence police) their place, but certainly it was a demonstration of power and deterrence."

Eerik-Niiles Kross, a former Estonian intelligence chief and diplomat, told AFP Kohver's arrest was unprecedented "during the entire post-Cold War era".

"In my opinion, it is not only an extraordinary event in Estonia but also much wider in the relationship between Russia and NATO," he added.

Obama reassured NATO's Baltic allies last week that the alliance and the US would guarantee their independence and protection in any fight with Russia.

NATO itself also announced a new rapid reaction force aimed at countering any threat to its members at a key summit last week in Wales.

Estonia, Latvia and Lithuania emerged from nearly five decades of Soviet occupation in the early 1990s and joined NATO and the European Union in 2004 in a bid to shore up their security amid tense relations with Moscow.

'Outrageous'

According to President Toomas Hendrik Ilves, the "abduction occurred in the course of a cross-border corruption investigation on Estonian soil. Russia admitted this was the work of their special services."

Russia's FSB claims it detained Kohver in northwestern Russia close to the Estonian border as he attempted to carry out an "undercover operation".

Moscow has so far failed to officially update Estonian authorities on the incident and diplomats has been unable to see Kohver, according to foreign minister Urmas Paet.

The incident has Estonian press and social media brimming with speculation.

"The kidnapping was aimed at undermining Estonia's sense of security," Postimees, the country's leading broadsheet daily, said in a Monday editorial.

It also claimed Kohver may have discovered Russian security officials' involvement in smuggling before he was detained.

"The question remains how did FSB operatives know about his presence at a given point and a given moment," Postimees said.

Arnold Sinisalu, head of the KAPO anti-corruption investigation unit, confirmed Kohver was "collecting information about cross border customs corruption in the widest sense".

For Estonians who still remember Soviet times, the incident conjured up bad memories of an era they hoped was consigned to history.

"The most disgusting thing with this kidnapping is the similarity to the actions of the inhumane dictatorship during the Soviet Union," Tallinn-based travel agent Juta Aavik, 40, told AFP.

"If they (Russia) want to, they arrest a person, fabricate whatever accusation, break all the rules and no one even coughs. Now it's a citizen of an independent country outside Russian territory. It's outrageous."