The White House says Iran has deployed military experts to Russian-held Crimea to help with drone strikes on Ukraine.
A US spokesman said the Iranians were trainers and tech support workers.
Ukraine’s capital Kyiv was hit on Monday by so-called “kamikaze” drones, deployed by Russia but believed to be Iranian-made.
Britain has announced sanctions against Iranian businesses and individuals responsible for supplying drones.
“We believe that Iranian military personnel were present in Crimea and assisted Russia in these operations,” White House national security spokesman John Kirby told reporters.
He said a “relatively small” number of Iranians were providing technical support and Russians were piloting drones in Ukraine.
“Tehran is now engaged directly on the ground, and through the delivery of weapons that are affecting civilians and civilian infrastructure in Ukraine,” Mr Kirby said.
He added that the U.S. “will take all possible measures” to expose, stop and counter Iran’s supply of these weapons against the Ukrainian people.
Ukraine on Monday identified 136 drones – or unmanned aerial vehicles (UAVs) – used as weapons by the Iranian Shahid.
They are known as “kamikaze” drones because they are destroyed in attack – named after the Japanese fighter pilots who flew suicide missions in World War II.
Russia has used drones and missiles to target critical infrastructure around Ukraine in recent days, destroying about a third of the country’s power stations since Monday last week.
As a result, restrictions on the use of electricity were introduced in Ukraine for the first time on Thursday.
