Iran has sent trainers to Ukraine to help Russians overcome problems with the fleet of drones that they bought from Tehran, according to The New York Times.
The US newspaper cited current and former officials who were briefed on the classified intelligence.
The Iranian trainers are operating from a Russian military base in Crimea, where many of the drones have been based since being delivered from Iran, the officials told the newspaper.
They said the trainers were from the Islamic Revolutionary Guard Corps, a branch of the Iranian military designated as a terrorist organisation by the US.
When Iran sent the first batch of drones to Russia, errors by Russian operators rendered them ineffective, the report said.
Mechanical issues also grounded the aircraft and limited their utility and Iran opted to send its trainers to Crimea.
In recent weeks, Ukrainian forces released images of what analysts say is the wreckage of Iran-made Shahed-136 drones, fired at targets across the country.
They have a distinctive V-shape and tail fins that often survive the explosive impact.
The so-called “kamikaze drones” — named because their attacks mimic Japanese suicide attack aircraft in the Second World War — are cheap and can be used in large swarms to overwhelm enemy air defences.
Reuters, meanwhile, reported that Iran has promised to provide Russia with surface-to-surface missiles, in addition to more drones, citing two senior Iranian officials and two Iranian diplomats.
A deal was agreed on October 6 when Iran's First Vice President Mohammad Mokhber, two senior officials from Iran's powerful Revolutionary Guard and an official from the Supreme National Security Council visited Moscow for talks with Russia about the delivery of the weapons.
"The Russians had asked for more drones and those Iranian ballistic missiles with improved accuracy, particularly the Fateh and Zolfaghar missiles family," one of the Iranian diplomats said.
A western official told Reuters an agreement between Iran and Russia would provide surface-to-surface short range ballistic missiles, including the Zolfaghar.
One of the drones Iran agreed to supply is the Shahed-136.
Fateh-110 and Zolfaghar are Iranian short-range surface-to-surface ballistic missiles capable of striking targets at distances from 300 kilometres to 700km.
The Iranian diplomat rejected assertions by western officials that such transfers breach a 2015 UN Security Council resolution.
"Where they are being used is not the seller's issue. We do not take sides in the Ukraine crisis like the West. We want an end to the crisis through diplomatic means," the diplomat said.
Missiles a 'logical next step'
Asked if Russia had used Iranian drones in its campaign in Ukraine, Kremlin spokesman Dmitry Peskov said the Kremlin did not have any information about their use.
"Russian equipment with Russian nomenclature is used," he said. "All further questions should be directed to the defence ministry."
A European diplomat told Reuters it was his country's assessment that Russia was finding it more difficult to produce weaponry for itself given the sanctions on its industrial sector and so was turning to imports from partners such as Iran and North Korea.
"Drones and missiles are a logical next step," the European diplomat said.
Asked about sales of Iranian surface-to-surface missiles to Russia, a senior US military official said: "I don't have anything to provide at this time in terms of whether or not that is accurate."
Iran denies using drones
Several European Union states on Monday called for sanctions against Iran over its supply of drones to Russia, as the bloc agreed a separate set of sanctions over Tehran's crackdown on unrest.
Tehran officially said on Tuesday said it was ready for talks with Kyiv to clarify claims that Iran is providing Russia with weapons and drones to be used in the war against Ukraine.
Calling the claims "baseless" and "based on false information", Iranian foreign ministry spokesman Nasser Kanani expressed "Iran's readiness for negotiation and discussion with Ukraine to resolve these accusations", according to a statement carried by AFP and state media.
"Claims made regarding the Islamic republic sending weapons including military drones to be used in the Ukraine war" are "untrue", according to the ministry statement.
Ukraine's Foreign Minister Dmytro Kuleba said on Tuesday he had suggested to President Volodymyr Zelenskyy that Kyiv cut diplomatic ties with Iran.
He cited "the amount of destruction Iranian drones have caused" in Ukraine and "the possible continuation of Iran's supply of weapons to Russia".