The United States says Russia's proposed sale of fighter jets to Iran would violate a UN arms embargo extended as part of the nuclear accord with Tehran.
The State Department said transferring the Sukhoi-30 planes requires UN Security Council approval. The planes are comparable to American F-15E fighter bombers.
State Department spokesman Mark Toner said on February 18 that Washington plans to raise the matter with Moscow and that the four other countries that negotiated the nuclear agreement "should be fully aware of these restrictions."
The deal kept the arms ban on Iran in place for another five years.
Toner didn't say if Washington will speak with Tehran about the possible purchase.