The transfer of ballistic missiles from Iran to Russia would signify a "dramatic escalation" of Tehran's support for Moscow, and the United States is prepared to respond with "significant consequences," the State Department said Monday.
US media outlets reported last week that Washington believed Iran had transferred the weapons to Russia for use in Ukraine, and the European Union has said allies shared "credible" intelligence that Tehran had done so.
"Any transfer of Iranian ballistic missiles to Russia would represent a dramatic escalation in Iran's support for Russia's war of aggression against Ukraine," State Department spokesman Vedant Patel told journalists.
"We have been clear... that we're prepared to deliver significant consequences," he said.
Tehran has rejected the accusation that it transferred the missiles, but the Kremlin has not explicitly denied it.
Faced with punishing Western sanctions, Moscow has turned to Iran and North Korea for weapons supplies to keep its war machine going in Ukraine.
Ukraine says it has been attacked with Iranian-designed Shahed drones on an almost daily basis from Russia, and has found fragments of North Korean missiles on its territory.
The reported delivery of missiles to Russia comes as the Kremlin has once again stepped up its bombing campaign against Ukraine's key infrastructure ahead of winter.