"We assess that Iranian military personnel were on the ground in Crimea and assisted Russia in these operations," White House national security spokesman John Kirby told reporters.

Kirby said that the Iranians in Crimea were trainers and tech support workers, and that the Russians were piloting the drones, which have caused significant damage on Ukrainian infrastructure.

"Tehran is now directly engaged on the ground, and through the provision of weapons that are impacting civilians and civilian infrastructure in Ukraine," he said.

"The United States is going to pursue all means to expose, deter and confront Iran's provision of these munitions against the Ukrainian people."

"We're going to continue to vigorously enforce all US sanctions on both the Russian and Iranian arms trade," Kirby added.