A ballistic missile defence shield which the US has activated in Europe is a step to a new arms race, Russian President Vladimir Putin said on Friday, vowing to adjust budget spending to neutralise "emerging threats" to Russia.

The US switched on the $800mn missile shield at a Soviet-era base in Romania on Thursday saying it was a defence against missiles from Iran and so-called rogue states.

But, speaking to top defence and military industry officials, Putin said the system was aimed at blunting Russia's nuclear arsenal.

"This is not a defence system. This is part of US nuclear strategic potential brought onto a periphery. In this case, Eastern Europe is such periphery," Putin said.

"Until now, those taking such decisions have lived in calm, fairly well-off and in safety. Now, as these elements of ballistic missile defence are deployed, we are forced to think how to neutralise emerging threats to the Russian Federation," he said.

Coupled with deployment in the Mediterranean of US ships carrying Aegis missiles and other missile shield elements in Poland, the site in Romania was "yet another step to rock international security and start a new arms race", he said.

Russia would not be drawn into this race. But it would continue re-arming its army and navy and spend the approved funds in a way that would "uphold the current strategic balance of forces", he said.

US Deputy Defence Secretary Robert Work said on Thursday that the shield would not be used against any future Russian missile threat.

Frank Rose, deputy US assistant secretary of state for arms control, warned at the time that Iran's ballistic missiles could hit parts of Europe, including Romania.

Putin said the prospect of a nuclear threat from Iran should no longer be taken seriously and was being used by Washington as an excuse to develop its missile shield in Europe.

The full defensive umbrella, when complete in 2018 after further development in Poland, will stretch from Greenland to the Azores.

It relies on radars to detect a ballistic missile launch into space. Sensors then measure the rocket's trajectory and destroy it in space before it re-enters the earth's atmosphere. The interceptors can be fired from ships or ground sites.