Russia has received its first batch of Iranian drones, according to reports, with plans to import hundreds more in the coming months.
Russian cargo planes flew in the Mohajer-6 and Shahed-series drones from airfields in Iran earlier this month, US officials told NBC News, which will likely be used in the invasion of Ukraine.
Ukraine has deployed Turkish drones against Russia, playing a key role in the country's fightback against the invasion.
Moscow is now looking to harness Iran's expertise in drone technology to make up for its own lack of militarised Unmanned Aerial Vehicles (UAVs).
It plans to ship in "hundreds" more Iranian drones, US officials said, in a bid to target positions behind Ukrainian enemy lines.
However, the sources said that Russia has struggled with numerous technological failures from the new weaponry, meaning the drones are unlikely to be deployed to the frontlines any time soon.
While Tehran has denied or not commented on the claims that it has provided drones to Moscow, US officials said in July that Russian officers had inspected UAVs at airfields in Iran.
The Iranian Revolutionary Guard Corp's aerospace commander Brigadier General Amir Ali Hajizadeh said earlier this month that the country was exporting drones abroad, but did not mention any client states.
Iran is thought to have supplied its proxies in Iran, Syria and Yemen with domestically produced drones and missiles.
The news comes as Russian Foreign Minister Sergey Lavrov met his Iranian counterpart Hossein Amirabdollahian on Wednesday, the latter handing over a proposal from an unnamed European country to end the war in Ukraine.
Russia invaded Ukraine on 24 February making early gains, but the offensive has largely ground to a halt after successful counter-attacks by Kyiv.
Ukraine's fleet of drones played a key role in holding off Moscow's assault, targeting Russian convoys behind enemy lines with pinpoint accuracy.
Such was the success of the Turkish-made Bayraktar that a song was made about the drone.