US army vehicles and soldiers in camouflage began crossing the German border into Poland on Thursday morning, as part of the largest US military reinforcement of Europe in decades.
President Barack Obama has pledged more than 3,000 soldiers to eastern Europe amid concerns among NATO allies of Russian aggression.
Kremlin spokesperson Dmitry Peskov said the move threatened Moscow’s “interests and our security”.
“This is even more pronounced when a third party (the US) reinforces its military presence on our doorstep in Europe,” he added.
Russian deputy foreign minister Alexei Meshkov described the operation as a “factor destabilising European security.”
From their base in Zagan in western Poland, US troops will move out to locations in Estonia, Latvia, Lithuania, Romania, Bulgaria and Hungary on a rotational basis.
The soldiers will “enhance deterrence capabilities in the region, improve the US ability to respond to potential crises and defend allies and partners”, an army spokesperson said.
The posting comes a week before the inauguration of Donald Trump, who has said he intends to thaw relations between Washington and Moscow.
Despite this, the President-elect’s pick for Secretary of State said NATO allies have a “right to be alarmed” about Russia.
On Wednesday, Rex Tillerson told the US Senate’s Foreign Relations Committee that Washington should have “an open and frank dialogue with Russia regarding its ambitions, so we know how to chart our own course”.