The posting of US troops in Poland is a threat to Russia's national security, the Kremlin has claimed.
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.
Outgoing US 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 was an "aggressive step" which represented a threat against Russia's national security.
From their base in Zagan, western Poland, US troops will move out to locations in Estonia, Bulgaria and Romania.
The 1,000-strong 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 nominee 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".
