Iran's military hosted two showcases for Russian delegations at the Kashan airfield, on June 8 and again on July 5, Sullivan said in a statement.

Satellite imagery released by the White House shows Shahed-191 and Shahed-129 drones at or flying near the site.

The White House said earlier this week that it believes Moscow is looking to acquire hundreds of the drones, or unmanned aerial vehicles (UAVs) -- and that Tehran is set to train Russian forces to use them as soon as this month.

"We assess an official Russian delegation recently received a showcase of Iranian attack-capable UAVs," Sullivan said Saturday.

"We are releasing these images captured in June showing Iranian UAVs that the Russian government delegation saw that day. This suggests ongoing Russian interest in acquiring Iranian attack-capable UAVs.

"To our knowledge, this is the first time a Russian delegation has visited this airfield for such a showcase," Sullivan added.

John Kirby, a spokesman for the national security council, told CNN this week that the drones can be used both for reconnaissance and to deliver munitions.

"It was important to make it clear to the world that we know that Russia needs these additional capabilities," Kirby said Tuesday. "They are expanding their resources at an accelerated rate."

The United States and allies have recently provided longer-range precision weapons to Ukrainian forces, like the Himars precision-guided missiles, boosting their capability to strike Russian targets and repel Moscow's intensifying attacks in the east.

Iran said Tuesday that "no special development" had taken place in technological cooperation with Russia following the invasion of Ukraine in February.

Drones have played a crucial role on both sides of the war in Ukraine, for everything from firing missiles from a distance, to dropping small bombs on targets, to conducting reconnaissance.

Ukraine's forces have had particular success in using Turkish-made Bayraktar armed combat UAVs, and the United States and other allies have supplied Kyiv with many types of smaller drones.