British Defense Secretary Michael Fallon says Russia poses a "real and present danger" to Estonia, Lithuania, and Latvia and NATO is preparing for possible attacks against the Baltic states.
Fallon said Russian President Vladimir Putin could use covert tactics to try to destabilize the three countries, former Soviet republics that are on NATO's eastern flank.
The Russian Foreign Ministry said on February 19 that Fallon's comments went beyond "diplomatic ethics" and that NATO countries were countering a "mythical Russian threat."
Ministry spokesman Aleksandr Lukashevich said NATO actually "poses threats that [Moscow has] to take into account in our military planning."
Fallon's comments, made during a trip to Africa on February 19, come one day after Britain had to scramble fighter jets to shadow two long-range Russian Bear bombers that were flying near England's southern coast in the second such incident this year.
The Defense Ministry said that in the latest incident, which happened late on February 18, Royal Air Force planes "escorted" the Russian planes out of the British "area of interest" and that at no time did the Russian aircraft go into British airspace.
Britain summoned the Russian ambassador in January after a similar event when Russian bombers flew over the English Channel.
Russian submarines were also detected north of Britain last year.
Fallon said he was "worried about Putin" and the way "he is testing NATO."
He said that NATO "has to be ready for any kind of aggression from Russia, whatever form it takes."
Fallon said Russia could replicate the tactics it used in Crimea and eastern Ukraine in the Baltics.
He noted Moscow's involvement in Ukraine, the bombers flying close to British airspace, and the detention last year of an Estonian security service agent by Russian forces.
"You have tanks and armor rolling across the Ukrainian border and you have an Estonian border guard being captured and still not returned [to Estonia]," he said.
"When you have [Russian] jets being flown up the English Channel, when you have [Russian] submarines in the North Sea, it looks to me like [the situation is]...warming up."
NATO-member country planes have intercepted Russian aircraft many times in the past year amid heightened tensions between Moscow and the West over Russia's involvement in the Ukraine crisis.
Fallon said earlier this month that Britain would send four Typhoon fighter jets to help NATO with policing flights in the Baltic states and promised up to 1,000 British troops for a NATO rapid-reaction force that is to be established.
Meanwhile, Lithuanian Foreign Minister Linas Linkevicius said on February 19 that Russia posed a threat not only to the Baltics but also to other countries, including Moldova.
He told BBC radio that "Russia is behaving aggressively now, as we speak."
But Linkevicius said NATO would react to any threat to Latvia's territorial integrity.
"If we fail to react properly to what's happening in Ukraine, there will be a big temptation [for Russia] to further instigate situations elsewhere and then we will face a bigger problem," he added.
Lithuanian President Dalia Grybauskaite and Estonian President Toomas Hendrik Ilves have also made public in recent months their concerns about Russia's aggressive behavior and the potential threat to the Baltic states.