Russian President Vladimir Putin said on Friday Moscow had no choice but to retaliate over the US missile shield in Europe, and warned that both Romania and Poland could find themselves in Russia's sights.

Some elements of the US missile shield, which Putin said was a direct threat to Russia's security, are being installed in Poland, and some in Romania.

"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 at a joint news conference in Athens with Greek Prime Minister Alexis Tsipras.

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

But he insisted that Russia was not taking the first step, only responding to moves by Washington. "We won't take any action until we see rockets in areas that neighbour us."-Reuters