Estonia and Finland plan to sign a coastal missile deal which they say would give them control of the Gulf of Finland, potentially severing Moscow's link with its fleet in the Baltic Sea.
The deal would rely on Estonia's new Israeli-Singaporean-built Blue Spear surface-to-sea missiles, which have a range of 180 miles.
Announcing the deal, Hanno Pevkur, Estonia's defence minister, said that the power dynamics of the Baltic region were changing, tipping towards Nato and away from Russia.
"The Baltic Sea will be Nato's internal sea when Finland and Sweden have joined Nato," he told the Finnish Iltalehti newspaper. "Compared with today, the situation is changing."
The Baltic Sea and the Gulf of Finland hold symbolic and strategic significance for Russians, who regard them as part of their military domain.
Without control of the Gulf of Finland, Russia's naval headquarters in St Petersburg cannot reach its Baltic Sea Fleet by sea.
It would only be able to access it from Kaliningrad, a Russian exclave wedged between Lithuania and Poland that Moscow captured during the Second World War.
The Kremlin hasn't commented on the preliminary agreement between Estonia and Finland, although Yury Shvytkin, deputy chairman of the Russian Duma's defence committee, said a threat to cut off the Gulf of Finland would trigger a Russian reaction.
"There are no barriers for Russian warships," he said. "We do not seek to escalate tensions however, such provocative statements require not only condemnation but also retaliatory actions on our part."
Military analysts have highlighted the Baltic Sea and the Gulf of Finland as a potential conflict zone. The US and Nato regularly hold naval exercises in the region.
Finland and Estonia are natural allies as their languages, kinship and cultures are similar.
Mr Pevkur said that they would integrate their coastal defence systems after Estonia receives its order of Blue Spear missiles from Israeli-Singapore joint-venture Proteus Advanced Systems. Finland's arsenal already includes MTO-85M surface-to-sea missiles.
"The flight range of Estonian and Finnish missiles is greater than the width of the Gulf of Finland," he said. "This means that we can connect our missile defences and share all our information with each other."
When Kalle Laanet, Estonia's then-defence minister, signed the deal to buy Blue Spear missiles last year he said that they were "one of the most hi-tech weapon systems of all time".
Juri Saska, the head of the Estonian Navy, said the weapon system "will form the cornerstone of Estonian naval defence for decades to come".
Finland and Estonia border the 250-mile-long Gulf of Finland along its northern and southern coast and their capitals, only 32 miles apart, guard its western mouth into the Baltic Sea.
Both countries also share land borders with Russia and regard Russians as their historic enemies.
Although the Kremlin's Baltic Sea Fleet is based mainly in Kaliningrad, its Navy is headquartered in the Admiralty in St Petersburg.
The warmongering 18th-century Tsar Peter the Great, a hero of Vladimir Putin, established the Russian Navy in St Petersburg and the Gulf of Finland, which ices over in winter, has always been its gateway to the Baltic Sea and the outside world.
Since the start of the war, Finland and former Soviet Estonia have given some of the strongest denouncements of Russia and Russian aggression. Their governments have said that even Russian tourists should be banned from entering the European Union.
Finland, as well as Sweden, were historically neutral but applied to join Nato after Russia invaded Ukraine in February.
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