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, reports BBC.
The US has seen reports that the two countries are considering joint production of lethal drones, he added.
Co-operation between Russia and Iran has been highlighted recently, with Ukraine accusing Russia of using Iranian drones in its attacks.
Iran initially denied sending any drones to Russia, but 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.
"In return, Russia is offering military and technical support to the Iranian regime, which will increase the risk it poses to our partners in the Middle East and to international security," he added.
He said the UK agreed with the US that Iranian support for the Russian military would grow in the coming months as Russia tries to get hold of more weapons, including hundreds of ballistic missiles.
Ukraine accused Iran of supplying Russia with "kamikaze" drones used in a series of attacks which killed at least eight people on 17 October.
Iran initially denied this, but later admitted sending a "limited number" of drones to Russia, "many months" before the war in Ukraine.
Ukraine's President Volodymyr Zelensky said this was a lie and that Ukraine was shooting down about 10 Iranian drones a day.