Russian President Vladimir Putin has hinted that American missile defence plans for Europe could spark an arms race.
His latest attack on the system, due to be deployed by 2020, came during a visit to an air base in southern Russia.
Washington insists it is intended to counter a potential threat from Iran not Russia.
Moscow says interceptors that the US and NATO are deploying will be able to destroy its own warheads in flight.
Putin said Moscow was ready to respond if necessary.
“The combat potential of any anti-missile system would be increased, and that means that we should react to this in due course. Of course it would be better for our partners not to do this, because actions that could lead to our strategic nuclear capability being devaluatued, will unavoidably provoke an appropriate reaction.”
“Whatever you call it, this has some elements of an arms race,” he added.
Putin has also ordered the military to speed up work on a new strategic bomber aircraft.
Earlier this month he called for legally-binding guarantees that the missile defence system will not be directed against Russia.
He is due to meet President Obama at the G20 summit next week.