President Vladimir Putin has escalated Russian criticism of a new US missile defence station in Romania, saying his country will "neutralise emerging threats".
He argued it was aimed at weakening Russia's nuclear power and vowed to increase Russian defence spending.
US President Barack Obama voiced concern about Russia's "growing aggressive military presence".
Nato says the base is aimed at potential threats from the Middle East.
The US on Thursday activated the estimated $800m (£550m; €700m) missile shield in Deveselu, southern Romania.
Nato Secretary General Jens Stoltenberg and other senior officials from the military alliance attended the opening ceremony, at a former Romanian air base.