(CNN) — 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.

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 Tuesday 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.

US officials have said that when Russia first began testing and deploying the drones in Ukraine in August, many of them experienced numerous failures. Russian operatives had been training on the systems inside Iran, but Iranian personnel began traveling to Crimea in recent weeks to help Russia operate the systems and try to fix their problems.

Tehran has provided two types: Shaheds, which explode on impact and have a range of upwards of 1,000 miles, and the Mohajer-6, which can both carry missiles and be used for surveillance.

US officials have seen Russia’s reliance on these Iranian drones — in particular the Shaheds — as evidence that Russia is struggling to replenish its native stocks of munitions after eight months of missile salvos and a punishing regime of Western sanctions that the US believes has cut Moscow off from needed components for new weapons. Iran has denied sending the drones to Russia.

Patel said that the United States would “continue to take practical, aggressive steps to make these weapons sales harder, including sanctions, export control actions against any entities involved.”

A US official told CNN that on Wednesday, the US, France and the United Kingdom plan to discuss Iran’s drone transfers to Russia during a closed UN Security Council meeting.

The US, France, and the UK have said that the transfer of the Iranian-made drones to Russia violates UN Security Council Resolution 2231, which restricts certain arms transfers to or from Iran. It is unclear whether they will raise this specific point in the meeting Wednesday or move to snap back sanctions on Iran for the arms transfers.