NATO leaders, during their annual summit in Chicago, issued a proclamation saying the "appropriate mix of nuclear, conventional and missile defense capabilities" was vital for the security of the alliance.
Washington and its allies say a missile defense shield in Europe would counter regional aggression for adversaries such as Iran. Russia says it views the plan as a threat to its national security, warning it may take corresponding action along its western border.
Grigory Berdennikov, the Russian envoy to the International Atomic Energy Agency, said it was tough for the Kremlin to move ahead with arms reductions while Washington was advancing missile defense plans in Europe.
"We cannot do anything else while there is no clarity on missile defense issues," he was quoted by Russia's state-run news agency RIA Novosti as saying.
Moscow confirmed Friday that it gave its consent for NATO to use the Russian airbase in Ulyanovsk as a transit hub for operations in Afghanistan.