"Russian transport aircraft loaded the UAV equipment at an airfield in Iran and subsequently flew from Iran to Russia over several days in August," said Pentagon spokesman Brigadier General Pat Ryder, using the initials for unmanned aerial vehicle.

"It's likely part of Russia's plans to import hundreds of Iranian UAVs of various types," he said.

However, Ryder said, "our information indicates that UAVs associated with this transfer have already experienced numerous failures."

He did not offer evidence for that claim.

After both sides have deployed and lost large numbers of surveillance and attack drones over the six months of the war, Russian are importing two types from Iran: the Mohajer-6 and the Shahed-series UAVs, according to the Pentagon.

Russian forces intend to use the Iranian UAVs for air-to-surface attacks, electronic warfare, and battlefield targeting.

Ryder said Moscow turned to Iran in part because sanctions and export controls directed at Russia by Ukraine's allies have made it harder for Russian industry to produce their own.