Top German, French, British and US diplomats wrote to the UN Secretary General on Friday urging the UN to investigate the use of Iranian drones by Russia in Ukraine.

They provided with their conclusion that the transfer violates a UN Security Council resolution which restricts certain arms transfers to or from Iran.

“We would welcome an investigation by the UN Secretariat team responsible for monitoring the implementation of UNSCR 2231 and stand ready to support the work of the Secretariat in conducting its technical and impartial investigation,” the diplomats wrote in a letter to UN Secretary General António Guterres.

UNSCR 2231 was tied to the 2015 Iran nuclear deal, and elements of that resolution, including a ban on the transfer of conventional weapons, were phased out in 2020. But the countries said that both Iranian drone models were manufactured after the resolution entered into force and that the transfer “has not been permitted in advance by the Security Council,” reads the letter.

Iran has denied that it supplied Russia with drones for use in Ukraine. The Kremlin has also rejected using Iranian weapons.