Washington alleges Moscow "is offering Iran an unprecedented level of military and technical support — that is transforming their relationship into a fully fledged defense partnership."
Russia is moving to provide advanced military assistance to Iran, including air defence systems, helicopters and fighter jets, US has said, part of deepening cooperation between the two allies as Tehran provides drones to Moscow that the West alleges are being used to support Russian offensive in Ukraine.
White House National Security Council spokesperson John Kirby on Friday cited US intelligence assessments for the allegations, saying Russia was offering Iran "an unprecedented level of military and technical support that is transforming their relationship into a full-fledged defence partnership."
Kirby alleged Russia and Iran were considering standing up a drone assembly line in Russia for the Ukraine conflict, while Russia was training Iranian pilots on the Sukhoi Su-35 fighter and Iran could receive deliveries of the plane within the year.
"These fighter planes will significantly strengthen Iran's air force relative to its regional neighbors," Kirby said.
Kirby said the arms transfers were in violation of UN Security Council resolutions and that the US would be "using the tools at our disposal to expose and disrupt these activities."
Tehran did not immediately comment on the Western allegations.
'Unprecedented military support'
The Biden administration has repeatedly accused Iran of assisting Russia with its invasion of Ukraine.
The administration says Tehran sold hundreds of attack drones to Moscow over the summer.
Kirby on Friday reiterated the administration's belief that Iran is considering the sale of hundreds of ballistic missiles to Russia, but acknowledged that the US doesn’t have "perfect visibility into Iranian thinking on why" the deal hasn't been consummated.
Britain's UN ambassador Barbara Woodward on Friday accused Russia of attempting to obtain more weapons from Iran, including hundreds of ballistic missiles, in return for "an unprecedented level of military and technical support" to Tehran.
"We are concerned that Russia intends to provide Iran with more advanced military components, which will allow Iran to strengthen their weapons capability," she said. "So it is imperative that the truth about Iran's supply to Russia is exposed, and is investigated by the UN as soon as possible."
At a UN Security Council meeting called by Russia to assess the impact of Western weapons pumped into Ukraine, Russia's UN Ambassador Vassily Nebenzia again denied that Iran is supplying weapons to Russia for use in Ukraine.
"The military industrial complex in Russia can work perfectly fine and doesn’t need anyone’s assistance, whereas the Ukrainian military industry does not basically exist and is being assisted by the Western industry and Western companies," he said.
UK Foreign Secretary James Cleverly called the Iran-Russia collaboration a "desperate alliance."
"Iran is now one of Russia's top military backers,"he said. "Their sordid deals have seen the Iranian regime send hundreds of drones to Moscow, which have been used to attack Ukraine’s critical infrastructure and kill civilians.
"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."
Even as the White House has accused Iran of backing Russia's war effort, the administration has not abandoned the possibility of reviving the 2015 Iran nuclear deal — scuttled by the Trump administration in 2018.