WASHINGTON - The United States stated on Wednesday that the missile defense system being deployed in Europe does not and cannot threaten Russia's strategic deterrent.

"We've been open and transparent with Russia on our plans for missile defense for a long time. We believe our missile defense reflects the growing threat to our allies from Iran that we're committed to deterring," State Department spokesman Mark Toner said.

"We've been clear all along for many years now that this system is not directed against Russia," he told reporters at a regular press briefing. "In multiple channels we've explained to Russian officials that the missile defense systems being deployed in Europe do not and cannot threaten Russia's strategic deterrent."

The spokesman was responding to threats by Russian President Dmitry Medvedev to station contemporary missile systems in the western and southern sections of Russia in response to any additional US missile deployments in Europe.

The president also warned on Wednesday that Russia may withdraw from the new Strategic Arms Reduction Treaty (START) with the United States, grumbling about the disregard of Moscow's demand for written, legal guarantees that the US missile shield in Europe will not target Russia.

Toner argued that the new START, which came into force in February 2011 and was seen as a tangible result of the United States' reset in relations with Russia, benefits the security and stability of both countries.

"Its implementation is going well, and we see no basis for threats to withdraw from it," he said.

"We don't see any reason for Russia to take any military countermeasures to missile defenses that won't affect the strategic balance between the US and Russia," he added.

The missile defense shield, which will be deployed in stages from 2011 to 2020 and involves the deployment of US interceptor missiles and radar in Europe, will be capable of intercepting long- range and intercontinental ballistic missiles.