Iran has sent military personnel to Russian-occupied territory inside Crimea to train and advise the Russian military on the use of Iranian-built drones that Moscow has used to devastating effect in its war in Ukraine, according to two sources familiar with US intelligence, writes CNN.

Russia has launched many of what is believed to be a store of hundreds of Iranian-made drones from Crimea in a fusillade that has targeted Ukrainian cities and energy infrastructure in increasing numbers in recent weeks. The drones have been seen as a signal of growing closeness between Tehran and Moscow.

CNN has reached out to the Iranian mission at the United Nations for comment.

State Department principal deputy spokesperson Vedant Patel said that the “deepening” of relations between Moscow and Tehran should be seen as “a profound threat”.

The Daily Mirror first reported the trainers’ presence in Ukraine.

It was not immediately clear how many trainers traveled to Crimea and whether they remain present. One source briefed on US intelligence said “dozens” of Iranian personnel had been sent.