Moscow hopes Washington “understands that Russia will be forced to react if the U.S. speeds up the placement of missile defense systems in Europe,” Russian Deputy Foreign Minister Sergei Ryabkov said Friday, May 23, according to RIA Novosti.

“We can’t remain indifferent to any attempt that affects Russia’s national potential of its strategic nuclear restraint,” Ryabkov said.

Late last month, the U.S. Navy deployed for the first time an advanced version of a missile-interceptor for its Aegis missile defense system, initiating the second phase of Washington’s plan to boost missile defenses in Europe.

The U.S. missile defense system in Europe, which NATO and the U.S. say is aimed at countering threats from North Korea and Iran, has been a particular source of friction in U.S.-Russian relations for a number of years.

Russia and NATO formally agreed to cooperate on the system at the 2010 NATO summit in Lisbon, but talks foundered, in part over Russian demands for legal guarantees that the system would not target its strategic nuclear deterrent.