The United States is sending an additional 2,000 troops to Europe and redeploying 1,000 already stationed in Germany to bolster the security of its NATO allies.
Russia has described the deployments as "destructive" and accused Washington of aggravating the crisis.
"We are constantly calling on our American partners to stop fuelling tensions," said Kremlin spokesman Dmitry Peskov, judging that "Russia's concern is clear and perfectly justified", given these are " Americans who are sent to European countries".
The majority of the 2,000 additional troops to be flown in from Fort Bragg, North Carolina, will be deployed in Poland, with about 300 heading for Germany.
Meanwhile, 1,000 troops already stationed in Germany will be sent to Romania.
Furthermore, 8,500 forces in the US remain in "heightened readiness" and therefore "ready to move" to support the NATO Response Force if needed.
The US Department of Defense said that they "continue to review our force posture and the situation in Europe, as the gravity of this situation demands our full attention".
During a press briefing, Pentagon spokesman John Kirby added that this sends a "clear signal" that the US is "not going to tolerate aggression" against its allies.
The troops' movements come amid tensions with Russia. Washington accuses Moscow of being on the brink of invading Ukraine after amassing hundreds of thousands of soldiers and military equipment along their shared border over the previous few months.
The Kremlin rejects the accusation but has issued a list of demands, including a guarantee that Ukraine will never join the security alliance. It has also accused the US of stoking "hysteria" over Ukraine.
NATO allies have categorically rejected Russia's demand that Ukraine not be allowed to join that alliance and its claim that the former Soviet state is part of its "sphere of influence".
Washington and European capitals have also repeatedly warned that any Russian military incursion into Ukraine would have "costly" consequences, warning of harsh economic sanctions.
Russian Deputy Foreign Minister Alexander Grushko said the "unfounded destructive steps" announced by the US on Wednesday, "will only fuel military tensions and narrow the field for political decisions,” in remarks carried by the Interfax news agency.
The US troops deployed in Poland come on top of the 4,000 already in the country.
The eastern European country, which borders Belarus — where Moscow has moved troops — is also home to NATO's Enhanced Forward Presence (EFP) units.
About 5,000 NATO troops have been deployed in Poland and the three Baltic countries — Estonia, Latvia, Lithuania — through this format since 2017. The four EFP battlegroups were created following Russia's illegal annexation of Crimea in 2014.
Romania is also home to a NATO permanent base and also has 1,000 US soldiers stationed in the country as part of a bilateral agreement.
Defence ministers from NATO member states are scheduled to meet in a couple of weeks to decide whether to expand the Enhanced Forward Presence to its south-eastern border in countries including Slovakia, Hungary, Romania and Bulgaria.