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, says US national security council spokesman John Kirby.
The US has seen reports that the two countries are considering joint production of lethal drones, he adds.
It comes after Ukraine accused Iran of supplying Russia with “kamikaze” drones used in deadly attacks on 17 October, 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 strikes left more than 1.5 million people without power in Odesa, Mr Zelensky said in his evening address, adding it could take days for electricity to be restored.
Earlier, 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 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 tried to get hold of more weapons, including hundreds of ballistic missiles.
On Saturday, Penny Wong, Australia’s foreign minister, said in a statement: “The supply of drones to Russia is evidence of the role Iran plays in destabilising global security. This listing highlights that those who provide material support to Russia will face consequences.”
She also announced measures against 19 other people and two entities, including Iran’s Morality Police, for the brutal treatment of anti-government protesters following the death of 22-year-old Mahsa Amini in custody earlier this year.
