The US says the two allies are giving each other ‘unprecedented support’ in Ukraine.
Iran is now Russia’s biggest military backer.
That’s the warning from the US National Security Council.
John Kirby, the national security spokesman, says the Ukraine war is pulling the two countries into what he calls a “full-fledged” defence partnership.
The West has accused Russia of using Iranian drones to kill civilians and destroy Ukrainian energy facilities.
Kirby believes Tehran and Moscow could start making these weapons together.
In return, he says, Russia is providing Iran with air defence systems, helicopters and training.
So, what are the implications for regional, and global, security?