The Kremlin said on Thursday it was concerned by what it described as a U.S. military build-up in Poland, saying the move represented a threat to its national security.
Addressing reporters on a conference call, Kremlin spokesman Dmitry Peskov said Russia regarded the move as an aggressive step along its borders.
A convoy carrying U.S. soldiers was welcomed on Poland's border crossing with Germany on Thursday as part of a NATO build-up that has drawn criticism from Russia.
A symbolic exchange of flags took place, as a German flag previously hoisted on the convoy's lead vehicle was replaced with a Polish one.
Throughout the week, troops, tanks, trucks and other military equipment have been arriving in Poland by rail and air, after arriving in Germany at the weekend.
Two shiploads arrived in the northern German port of Bremerhaven before heading to Poland as fleets of tracked and wheeled vehicles which will be used by around 4,000 U.S. troops being deployed for exercises in NATO states near Russia.
U.S. and Polish forces will participate in a large "massing" exercise in Poland at the end of January as part of a series of measures aimed at reassuring U.S. allies in Europe after Russia's 2014 annexation of the Crimea region of Ukraine.
NATO countries say their planned deployments to eastern NATO countries are purely defensive, but Russia has rebuked what it sees as an aggressive western build-up in eastern Europe.
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