Russian President Vladimir Putin said Russia will retaliate against the placement of U.S. missiles in nearby countries such as Romania, according to Russia's state-run news agency TASS.
The United States launched a ground-based missile defense system earlier this month in Romania.
Putin issued his threat during a news conference Friday after meeting with Greek Prime Minister Aleksis Tsipras in Athens.
"If yesterday in those areas of Romania people simply didn't know what it means to be in the cross hairs, then today we will be forced to carry out certain measures to ensure our security," Putin said.
NATO, which operates the missile defense system, said the missiles could not be used offensively as they don't include explosives and are designed to simply "punch" targets out of the sky.
The NATO run system is housed at a U.S. naval support facility in Deveselu, Romania, the site of a Romanian military base. An additional anti-missile platform is planned in Poland.
"It will be the same case with Poland," Putin warned. "We will wait until Poland takes certain action. We won't take any action until we see rockets in areas that neighbor us."
The U.S. is making the absurd claim that the system is meant to defend Europe against rogue states like Iran and not intended to target Moscow's missiles.
In 2008, then-president Dmitiry Medvedev said "Russia will deploy Iskander missile systems in its enclave in Kaliningrad to neutralize, if necessary, the anti-ballistic missile system in Europe."