The White House said on Friday that Russia appeared to be deepening its defense cooperation with Iran and had received hundreds of one-way attack drones that it is using to strike Ukraine.

Citing newly declassified information, the White House said the drones, or Uncrewed Aerial Vehicles (UAVs), were built in Iran, shipped across the Caspian Sea and then used by Russian forces against Ukraine.

“Russia has been using Iranian UAVs in recent weeks to strike Kyiv and terrorize the Ukrainian population, and the Russia-Iran military partnership appears to be deepening,” White House spokesman John Kirby said in a statement.

“We are also concerned that Russia is working with Iran to produce Iranian UAVs from inside Russia.”

Kirby said the U.S. had information that Russia was receiving materials from Iran required to build a drone manufacturing plant that could be fully operational early next year.

“We are releasing satellite imagery of the planned location of this UAV manufacturing plant in Russia’s Alabuga Special Economic Zone,” he said.

The U.S. has previously sanctioned Iranian executives at a defense manufacturer over drone supplies to Russia. Iran has acknowledged sending drones to Russia but said in they past they were sent before Russia’s February invasion. Moscow has denied its forces used Iranian drones in Ukraine. A White House official said Iran had transferred several hundred drones to Russia since August.

Support between Iran and Russia was flowing both ways, Kirby said, with Iran seeking billions of dollars worth of military equipment from Russia including helicopters and radars.

“Russia has been offering Iran unprecedented defense cooperation, including on missiles, electronics, and air defense,” he said.

“This is a full-scale defense partnership that is harmful to Ukraine, to Iran’s neighbors, and to the international community. We are continuing to use all the tools at our disposal to expose and disrupt these activities including by sharing this with the public – and we are prepared to do more.”

Kirby said the transfers of drones constituted a violation of United Nations rules and the United States would seek to hold the two countries accountable.

Britain, France, Germany, the U.S. and Ukraine say the supply of Iranian-made drones to Russia violates a 2015 U.N. Security Council resolution enshrining the Iran nuclear deal.

Under the 2015 U.N. resolution, a conventional arms embargo on Iran was in place until October 2020.

Ukraine and Western powers argue that the resolution includes restrictions on missiles and related technologies until October 2023 and can encompass the export and purchase of advanced military systems such as drones.

The Iranian and Russian missions to the United Nations did not immediately respond to a request for comment on the U.S. accusations.

“We will continue to impose sanctions on the actors involved in the transfer of Iranian military equipment to Russia for use in Ukraine,” Kirby said.

He said a new U.S. advisory issued on Friday aimed “to help businesses and other governments better understand the risks posed by Iran’s UAV program and the illicit practices Iran uses to procure components for it.”

The advisory highlighted key items sought by Iran for its development of drones, including electronics such as processors and controllers.