Russia has deployed nuclear-capable Iskander missiles in its western-most region, Kaliningrad, which borders on Nato members Poland and Lithuania.
Poland said the development was of the "highest concern", adding it was monitoring the situation.
Russia's defence ministry said the new deployment was part of military exercises and had happened before.
The US and Nato have seen disagreements with Russia intensify in recent times, particularly over Syria and Ukraine.
Kaliningrad is a Russian enclave sandwiched between Poland and Lithuania.
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The Iskander system has a range of up to 700km (440 miles) and could reach the German capital, Berlin.
Polish Defence Minister Antoni Macierewicz called Russia's activities "very alarming".
And a US intelligence official told Reuters the move could be to express displeasure at Nato. Nato is boosting its eastern flank by deploying four battalions in Poland, Lithuania, Latvia and Estonia next year.
