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.
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”.
Nato sought to soothe those fears at its Warsaw summit in July by announcing it would deploy troops to both the Baltic states and Poland, our correspondent says.
Full article: Russia deploys nuclear-capable missiles in Kaliningrad (BBC)