The Iranian Revolutionary Guard is believed to be on the ground in Crimea helping Russian forces operate drones, sparking fresh tensions over the conflict.
“Kamikaze” drones deployed by Russia but believed to be Iranian-made have devastated Ukraine’s power infrastructure in recent days.
US officials believe Iran may have deployed military personnel to assist the Russians, in part because of their lack of familiarity with the Iranian-made drones.
“We can confirm that Russian military personnel based in Crimea have been piloting Iranian UAVs and using them to conduct kinetic strikes across Ukraine, including in strikes against Kyiv in recent days,” US State Department spokesman Ned Price told a daily briefing with reporters, referring to unmanned aerial vehicles, or drones.
“We assess that ... Iranian military personnel were on the ground in Crimea and assisted Russia in these operations,” Mr Price said. He said “we do have credible information” but he did not provide evidence.
Russia seized Crimea from Ukraine in 2014 and has used the southern peninsula to train soldiers and reopen Soviet-era military bases as part of the invasion of its neighbour.
There was no immediate public reaction from Tehran to the US accusations but Iran has denied the drones are Iranian-made. Russia has also denied using Iranian drones in Ukraine.
“Iran and Russia, they can lie to the world, but they certainly can’t hide the facts, and the fact is this: Tehran is now directly engaged on the ground,” White House national security spokesperson John Kirby told reporters without providing details.
Ukrainian citizens endured the first day of nationwide scheduled power outages since the war began eight months ago so repairs could be made to damaged or destroyed energy plants as winter approaches.
It came as the UK announced new sanctions on Iranian officials and businesses accused of supplying the drones.
Foreign Secretary James Cleverly said: “These cowardly drone strikes are an act of desperation. By enabling these strikes, these individuals and a manufacturer have caused the people of Ukraine untold suffering.
“We will ensure that they are held to account for their actions.”
Defence Secretary Ben Wallace held emergency talks in Washington amid increasing concerns over the threat of a nuclear attack.
Mr Wallace pulled out of an appearance before the Commons defence committee on Tuesday so he could fly to Washington for the talks at the Pentagon and White House.
They are understood to cover the latest situation in Ukraine, including Mr Putin dangling the threat of a nuclear attack, having already unleashed a wave of attacks on civilian infrastructure, including electricity plants, as his invasion suffers more military setbacks.