Russian President Dmitry Medvedev has warned that missiles could be deployed on the EU's borders if the US pursues its missile defence plans.
In a televised statement on November 23, he said "modern weapons systems" could be deployed in Kaliningrad if Russia, the US and Nato failed to come to a deal.
He added that Moscow may opt out of the New Start arms deal agreed with the US.
Washington wants an anti-missile shield ready by 2020 but Moscow considers the idea a threat to its nuclear forces.
The US says the shield is intended to provide protection from the potential missile threat posed by countries like Iran.
It had originally intended to locate major parts of its missile defence shield in Poland and the Czech Republic under Bush-era plans. But Russia had objected vigorously.
Presidents Barrack Obama and Medvedev signed the New Start nuclear arms treaty - to reduce their nuclear arsenals - in April 2010 and it was ratified by the US Senate last December.
The agreement was described by Mr Obama as the most significant in nearly two decades, so if Russia followed through on its threat to opt out it would be a big blow to relations.