The US State Department on Thursday claimed that Russian officials had trained in Iran in recent weeks as part of an agreement on a drone transfer from Tehran to Moscow.
The White House last month said that US intelligence had indicated Iran was preparing to provide Russia with hundreds of drones, including those that are weapon-capable.
It added that Russian officials had visited Iran to view the unmanned aerial vehicles (UAV).
The claim raised concerns that Iran, which has supplied drones to its allies in the Middle East, was now giving support to Russia for its war in Ukraine — a claim Iran's foreign minister denied.
Iran provided similar drones to Yemen's Houthi rebels, US National Security Adviser Jake Sullivan said at the time.
State Department deputy spokesman Vedant Patel told reporters during a phone briefing on Thursday that Russian officials had conducted training on drones in Iran “in the last several weeks”.
The US will “vigorously enforce” its sanctions on both Russian and Iranian weapons trading, he said, noting that the transfer of drones between the two countries was “potentially sanctionable under numerous authorities”.
“We remain incredibly concerned about Iran's use and proliferation of UAVs. They have been used to attack US forces, our partners in the region and international shipping entities,” Mr Patel said.
Reuters contributed to this report