The US national security council spokesman John Kirby says Russia is given an unprecedented level of military support.
The US has seen reports that the two countries are considering joint production of lethal drones, he adds, according to a BBC report.
This comes after Ukraine accused Iran of supplying Russia with “kamikaze” drones used in deadly attacks on October 17, which Tehran initially denied.
The Middle Eastern country later admitted sending Moscow a limited number of drones, “many months” before the war.
In response, Ukraine’s President Volodymyr Zelensky said this was a lie and that many more Iranian drones were being used.
The Ukrainian air force said it downed 10 of 15 such drones used to attack southern regions in the early hours of Saturday. The governor of Odesa said the strikes caused power cuts in most of his region.
Australia announced it was sanctioning three Iranians and one Iranian business for supplying Russia with drones to use against Ukraine.
Speaking on Friday, Mr Kirby said 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…” the BBC quoted him to have 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,” he added.
