Russian President Vladimir Putin has responded strongly to the US missile defense shield in Romania and Poland, as well as the Crimea issue. He made the comments at a joint news conference with Greek Prime Minister Alexis Tsipras in Athens on Friday, during his two-day official visit to Greece. This is his first trip to a EU country this year.

Again, Russia says it will take retaliatory measures against a US missile shield in eastern Europe. 

The missile defense site in Romania became operational earlier this month. And work is going ahead on another part of the shield, in Poland. US said the shield needed to protect from Iran, not aimed at Russia.

But Moscow considers the missile shield a threat to its security, arguing it can easily reach Russian cities. President Vladimir Putin warned Romania and Poland could find themselves in the sights of Russian rockets in result.

"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.

The Russian president also touched on the thorny issue of Crimea, ruling out any discussions on the region's status.

"As far as Crimea is concerned, we consider this question is closed forever. This was a historical decision taken by the people that live in Crimea. Russia will not conduct any discussions with anyone on this subject," Putin said.

Relations between Russia and the EU have been overshadowed since Crimea declared independence from Ukraine and was integrated into Russia in March 2014. 

The 28-nations bloc is on track to renew its sanctions against Russia over the annexation.

While Greece's Prime Minister, Alexis Tsipras, told the news conference that sanctions are not productive.

"The vicious circle of militarization, of Cold War rhetoric and of sanctions is not productive. The solution is dialogue. Because everybody can recognize that there is no future for Europe with EU and Russia in rivalry. So, Greece's usefulness is based on the fact that it can be a bridge of friendship and cooperation," Tsipras said.

Trade and economic cooperation, especially in the field of energy, is a focus of the two leaders talks. 

A number of bilateral agreements, including a deal between Russian oil giant Rosneft and Greek refiner Hellenic Petroleum are expected to be signed.