Russian officials trained in Iran in recent weeks as part of an agreement on the transfer of drones between the two countries, the U.S. State Department said on Thursday.

U.S. officials said last month that Washington had information that Iran was preparing to provide Russia with up to several hundred drones, including some that are weapons capable, and that Russian officials had visited Iran to view attack-capable unmanned aerial vehicles (UAVs).

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.

Iran's foreign minister at the time denied the claim, including in a phone call with his Ukrainian counterpart.

U.S. State Department deputy spokesperson 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 United States would "vigorously enforce" its sanctions on both Russian and Iranian weapons trading, he said. The transfers of drones between the two countries was "potentially sanctionable under numerous authorities," Patel said.

"We remain incredibly concerned about Iran's use and proliferation of UAVs. They have been used to attack U.S. forces, our partners in the region, and international shipping entities," Patel said.