Iran has become Russia's main military supporter alongside North Korea. The deal is apparently close to completion: Russia will soon acquire Fateh-110 and Zolfaghar short-range missiles from Tehran.
US officials fear that this would significantly expand the Russian army's military capabilities to attack targets in Ukraine at a critical phase of the war. Russia has recently appeared primarily as an importer of Iranian drones - while it has been hesitant about exporting military equipment to Iran. Russia's concern for geopolitical balance in the Gulf has prevented Moscow from supporting Tehran militarily.