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. The system is meant to defend Europe against rogue states like Iran and not intended to target Moscow's missiles, Washington has said.
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.
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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.
"Any threats from Russia are unjustified and irresponsible," said NATO spokeswoman Oana Lungescu. "Russia knows full well that our Ballistic Missile Defense system is defensive and neither designed nor directed against Russia."
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."
Putin has a track record of acting on this threats, so Washington and NATO have to take him seriously, military analyst Lt. Col. Rick Francona (ret.) told CNN.
"I don't think he wants to get into an armed confrontation with NATO," Francona said, "but of course Washington has to look at this. He is a very maniacal personality and we're not quite sure how he's going to react to this."
NATO Secretary General Jens Stoltenberg will visit Poland Monday and meet with Polish President Andrzej Duda.
The missile installation in Romania is the first land-based defensive missile launcher in Europe. It is part of a larger NATO defense shield that includes a command-and-control center at Ramstein Air Base, Germany, a radar installation in Turkey and four ships capable of identifying enemy missiles and firing their own SM-3s based in Rota, Spain.