[Russian President Putin meets with his Greek counterpart Pavlopoulos at the Presidental Mansion in Athens]Russian President Vladimir Putin said on Friday Moscow had no choice but to retaliate over the U.S. missile shield in Europe, and warned that both Romania and Poland could find themselves in Russia's sights. Some elements of the U.S. 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.