Russian President Dmitry Medvedev warned on Wednesday that Russia would have to build up its nuclear capability if NATO and the United States failed to reach an agreement with Moscow on European missile defense cooperation.

Medvedev and U.S. President Barack Obama are expected address the missile defense issue in late May at the G8 summit in Deauville, France.

"We believe that the NATO missile defense system and the Russian system should cooperate. But these should be two separate systems," Klikh said.

Russian Deputy Foreign Minister Sergei Ryabkov said on Monday that Moscow was concerned by the United States' refusal to provide legally binding guarantees that its European missile defense system would not be directed against Russia.

Moscow has warned it might pull out of the new START Treaty.

Russia and NATO agreed to cooperate on the so-called European missile shield during the NATO-Russia Council summit in Lisbon in November 2010. NATO insists there should be two independent systems that exchange information, while Russia favors a joint system.

Russia is opposed to the planned deployment of U.S. missile defense systems near its borders, claiming they would be a security threat. NATO and the United States insist that the shield would defend NATO members against missiles from North Korea and Iran and would not be directed at Russia.