Vladimir Putin has said Russia is considering "putting an end" to the "emerging threat" to its security from NATO's missile defence shield.
The Russian president dismissed US claims that the shield is not aimed against Russia and is instead designed to stop a missile threat from Iran.
Some of the missiles are already carried on ships in the Mediterranean, and others are based in Romania.
Mr Putin said he viewed the development as a threat to global security. Without offering details, he said Russia must take steps to strategically keep up with the West.
"Now that these anti-missile elements have been installed we will be forced to consider putting an end to the threats emerging in relation to Russia's security," AFP reported Mr Putin as saying.
Russia "will do everything needed to ensure and preserve the strategic balance, which is the most reliable guarantee from large-scale military conflicts", he added.
NATO's missile defence system was first proposed in 2002 but it has taken several years to get off the ground.
In 2009, the Obama administration said it would be deploying the sea-based Aegis weapon system on ships patrolling waters to stop any Iranian missiles.
The system became fully operational in 2012 when a radar system in Turkey was brought under NATO command.
The second stage went live on Thursday when an $800m US missile defence site in Deveselu, Romania, became operational.
CBS reported that Romania's President Klaus Iohannis said his country wanted NATO to have a "permanent naval presence" in the Black Sea and called for increased security for NATO members in the south and east, which border Russia and the Middle East.
Construction of another missile site in Redzikowo, Poland, was due to get under way on Friday and NATO is also planning to set up command and control centres in Latvia, Estonia, Lithuania, Poland, Romania and Bulgaria by the end of 2016.
Russia's consistent opposition to NATO's missile programme on its borders has come amid increased tensions in the region following the annexation of Crimea, accusations of Russian involvement in the conflict in eastern Ukraine and Russia's part in the Syrian conflict.
Both the US and the UK have sent military jets to several eastern European countries in what both countries say is a deterrence.