WASHINGTON: The United States on Friday expressed alarm over a "full scale defense partnership" between Russia and Iran, describing it as "harmful" to Ukraine, Iran's neighbors and the world.

Iran stands accused by Western powers of supplying drones to Russia for its war against Ukraine, as Moscow batters the country's energy infrastructure in search of an advantage in the bloody conflict.

"Russia is seeking to collaborate with Iran, in areas like weapons development, training," White House national security spokesman John Kirby told reporters.

US has not seen transfer of Iranian ballistic missiles to Russia: Kirby

Russia "is offering Iran an unprecedented level of military and technical support -- that is transforming their relationship into a fully fledged defense partnership," he told reporters.

"We have also seen reports that Moscow and Tehran are considering the establishment of a joint production line for lethal drones in Russia. We urge Iran to reverse course (and) not to take these steps."

Kirby said that the United States would sanction three Russian-based entities active in "the acquisition and use of Iranian drones."