US plans to permanently station US heavy military equipment on Nato's eastern flank would be the "most aggressive step by the Pentagon and Nato" since the end of the Cold War, a senior Russian defence official has warned.
Russia would "be left with no other option but to boost its troops and forces on the western flank", warned General Yuri Yakubov.
The Kremlin has declined to comment further on the plans.
The warning comes as Poland and Lithuania confirmed they are in talks with Washington about US heavy military equipment deployment close to the Russian border.
Both countries have been pressing the US to enhance its military presence in Eastern Europe, with their security concerns mounting following Russia's annexation of the Crimea and Moscow's support for the war in eastern Ukraine.
"There are talks about the stationing of US military equipment in Poland," said Tomasz Siemoniak, the Polish defence minister.
"We have been working for a while on increasing the American military presence in Poland and across the eastern flank of Nato," he continued, adding he expected an American decision "soon".
If the US goes ahead with the deployment it will be the first time it has committed equipment permanently to former Soviet-bloc countries. It could also run the risk of angering Moscow, which has long maintained a disputed claim that the West agreed not to station its armed forces in Eastern Europe following the collapse of Soviet rule in the early 1990s.
The equipment, which could include Abrams main battle tanks and Bradley armoured fighting vehicles, would be stockpiled in the region and kept ready for action while the troops remain on bases elsewhere.
"Lithuania is working to facilitate the deployment and stationing of heavy US equipment, and we are willing to donate our military infrastructure to house it," said Juozas Olekas, the Lithuanian defence minister. "We are in the process of preparing the military infrastructure for it. It is almost ready."
The confirmation of talks with Washington follows a story in the 'New York Times' outlining US plans to position equipment on the alliance's eastern flank.
Any deployment should provide Poles and Lithuanians tangible evidence of America's long-term commitment to their security at a time when jitters over Moscow's military might are increasing.
Linas Linkevicius, the Lithuanian foreign minister, said the US plan was not an aggressive act against Russia but were just "security enhancements".