Russia and Iran‘s relationship has warmed to a fully fledged defence partnership, the US has said.
Russia is giving an unprecedented level of military support, said US national security council spokesman John Kirby.
The US has seen reports that the two countries are considering joint production of lethal drones, he added.
Australia has announced it is sanctioning three Iranians and one Iranian business for supplying Russia with drones to use against Ukraine.
After initially denying sending any drones to Russia, the Middle Eastern country later admitted it had supplied some before the invasion of Ukraine.
Mr Kirby said that a partnership between Iran and Russia to produce drones would be harmful to Ukraine, Iran’s neighbours and the international community.
“Russia is seeking to collaborate with Iran in areas like weapons development, training,” he said, adding that the US fears that Russia intended to “provide Iran with advanced military components” including helicopters and air defence systems.
“Iran has become Russia’s top military backer…” he said. “Russia’s been using Iranian drones to strike energy infrastructure, depriving millions of Ukrainians of power, heat, critical services. People in Ukraine today are actually dying as a result of Iran’s actions.”
In response to Mr Kirby’s comments, UK Foreign Secretary James Cleverly said that Iran had become one of Russia’s main military supporters and that the relationship between them was threatening global security.
The “sordid deals” between the two countries have seen Iran send hundreds of drones to Russia, he said.