Romania and Poland are in the potential crosshairs of Russian rockets because they are hosting parts of the U.S. missile defense shield that Moscow considers a threat to its national security, President Vladimir Putin warned Friday.

Putin warned the European nations that Moscow has repeatedly said it would have to retaliate if the missile defense shield was put into place, but U.S. allies have ignored Russia’s warnings.

Earlier this month, the U.S. program was declared operational at a site in Romania, drawing an angry reaction from Russia. NATO said the system is purely defensive and a response to a growing capability of ballistic missiles globally.

Washington has said that the shield is needed to protect Europe from Iran and is not a threat. Another part of the shield is getting ready to be implemented in Poland.

"If yesterday in those areas of Romania people simply did not 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 in a joint news conference in Athens with Greek Prime Minister Alexis Tsipras.

"It will be the same case with Poland," he said.

Reuters reported that Putin didn’t lay out the actions that Moscow would take, but insisted that it was not making the first move. It was only going to respond to Washington’s moves.

"We won't take any action until we see rockets in areas that neighbor us,” Putin added.

Putin cried foul over the claim that the defense system was need to protect from Iran. He said that excuse made no sense because of the landmark nuclear deal that was struck to curb Tehran’s nuclear ambitions.

Putin said the missiles coming out of Europe could easily hit Russia.

"We've been repeating like a mantra that we will be forced to respond... Nobody wants to hear us. Nobody wants to conduct negotiations with us."

Putin has made only a handful of visits to EU countries since sanctions were imposed on Moscow two years ago in response to the Ukraine crisis and Russia's annexation of Crimea following an internationally disputed referendum.

"The issue of Crimea is over forever, based on the will of the people who live there. Russia will never negotiate on this issue," Putin said.

Later in the conference, Putin blamed the U.S. for and the European Commission for pressuring Bulgaria from shelving a gas line pipeline that would’ve gone under the Black Sea. He said Russia was going to go through with a similar project in the Baltic Sea.

On Saturday, Putin will visit the autonomous Orthodox Christian monastic community of Mount Athos, joined by the head of Russia's Orthodox Church, Patriarch Kirill of Moscow.

The Associated Press contributed to this report.

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