Acting Under Secretary of State Rose Gottemoeller said U.S. plans to deploy a global missile shield do not pose any threat to Russia's strategic nuclear forces.
Speaking in Moscow, Gottemoeller called for more dialogue and cooperation with Russia.
She said Russian and U.S. experts should discuss specific details about the missile-defense system in order to dispel Moscow's concerns.
Russia has rejected U.S. assurances that the system is needed to fend off an Iranian missile threat.
Moscow also has expressed concerns that the system eventually will grow powerful enough to challenge Russia's nuclear deterrent.
U.S. President Barack Obama this week defended private statements to his Russian counterpart that were picked up at the end of a one-on-one meeting in Seoul on missile defense and other sensitive topics.