Nato has accused Russia of "aggressive military posturing" following reports that it has deployed anti-ship missiles in its westernmost Baltic region.
Russia's Interfax news agency said on Monday that Bastion missile-launchers had been sent to Kaliningrad.
In a statement to the Associated Press, Nato said the move "does not help to lower tensions or restore predictability to our relations".
The Kremlin has accused Nato of stoking tensions by expanding eastwards.
Kaliningrad is a Russian exclave sandwiched between Poland and Lithuania.
In October, Russia sent nuclear-capable Iskander missiles to Kaliningrad, a move Poland described as of the "highest concern".
Russia said the deployment was part of military exercises and had happened before.
The Russian exclave of Kaliningrad represents an important military outpost between Poland and Lithuania with its coastline on the Baltic Sea.
The accumulation of radars and air defence systems, as well as coastal anti-shipping missiles, all form part of Moscow's developing "anti-access and area denial strategy", which in essence seeks to push Nato forces away from Russia and to make it very difficult to reinforce Nato members in the Baltic region in the event of a crisis.
However, in a separate statement on Monday, the RIA news agency quoted Russian defence committee chairman Viktor Ozerov as saying Iskanders and S-400 surface-to-air missiles were deployed in Kaliningrad to counter a planned US missile defence shield in eastern Europe.
The Bastion system fires Oniks cruise missiles, which have a range of up to 280 miles (450km). Russia has already used them in the Syrian civil war where it is supporting President Bashar al-Assad.Image copyrightEPAImage caption The deployment of the Iskander system in Kaliningrad has alarmed Poland
On Tuesday, Nato urged greater transparency on military activities "to avoid incidents and the risk of misunderstandings".
US state department spokesman John Kirby also called the latest Russian moves "destabilising to European security".
"Russia has made threats to move its Iskander missiles to Kaliningrad for the past decade in response to a variety of developments in Europe, none of which demand such a military response," he said.
"We call on Russia to refrain from words or deeds that are inconsistent with the goal of promoting security and stability."
His comments brought a stinging rebuke from the Russian defence ministry, which said that "all recent threats to European security" were a consequence of US military policy.