Russia has deployed nuclear-capable Iskander missiles in its Kaliningrad region, bordering Poland and Lithuania, BBC reports.
Linas Linkevicius, foreign minister of Nato-member Lithuania, said that the aim of the move was to "seek concessions from the West".
The US and Nato have seen disagreements with Russia intensify in recent times, particularly over Syria and Crimea.
Russia's defence ministry said the new deployment was part of military exercises and had happened before.
Kaliningrad is a Russian enclave sandwiched between Poland and Lithuania.
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.
The Iskander system could reach the German capital, Berlin.
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