The Russian president said Moscow would be ‘forced to carry out certain measures’ against the European nations, at an Athens conference on Friday.
Earlier this month the U.S. military – which says the shield is needed to protect from Iran, not threaten Russia – angered Russia by switching on the Romanian part of the shield.
Work is also going ahead on another part of the shield, 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 told a joint news conference in Athens with Greek Prime Minister Alexis Tsipras.
‘It will be the same case with Poland,’ he said.
Putin did not specify what actions Russia would take, but he insisted that it was not making the first step, only responding to moves by Washington.
‘We won’t take any action until we see rockets in areas that neighbour us.’
He said the argument that the project was needed to defend against Iran made no sense because an international deal had been reached to curb Tehran’s nuclear programme. The missiles that will form the shield can easily reach Russian cities, he said.
‘How can that not create a threat for us?’ Putin asked.
He voiced frustration that Russia’s complaints about the missile shield had not been heeded.
‘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.’