Russia says it wants legal guarantees from the United States that a proposed missile defense shield for Europe will not threaten Russian security.

Deputy Foreign Minister Sergei Ryabkov said Monday that Moscow has been disappointed by Washington's reluctance to provide such assurances, saying it cannot agree to missile defense cooperation with the U.S. and NATO without it.

Russia demanded defense safeguards from Washington earlier this month, after the U.S. reached an agreement to deploy anti-missile interceptors in Romania as part of the new defense shield. Moscow says the prospective missile defense system may pose a risk to Russian strategic nuclear forces in the future.

The U.S. says the system is aimed at countering possible missile attacks from countries such as Iran or North Korea, and is not aimed against Russia.

Russian President Dmitry Medvedev sent a letter to NATO heads of state in recent days, saying that a European missile defense system could only be viable if Russia takes part. 

Medvedev has warned that failure by Russia and Western nations to reach an agreement on the issue could trigger a new arms race.