The US State Department is claiming that the Russian use of an Iranian air base for strikes against ISIS in Syria could violate UN Security Council Resolution 2231, and claims to be consulting “our lawyers” to decide if it amounts to a violation.
Resolution 2231 bans Iran from obtaining nuclear-capable ballistic missiles or combat aircraft, though the State Department claims it can be used to do a lot of other things, and that Russia having military assets inside Iran might amount to the “transfer of military technologies” to them.
Russian officials are pointing out that’s obviously not the case, and that no aircraft or supplies are being transferred to Iran, they’re just using an Iranian base. The language of the resolution does not suggest any ban on Iran allowing the use of their air bases.
The US opposes Russia’s airstrikes in Syria, even though both are targeting ISIS. It is unsurprising, then, to see the US complain about the matter, though the resolution’s mention appears to be grasping at straws, as it clearly requires a broad interpretation of the text’s language, and it is unlikely anyone else on the Security Council is going to see things America’s way.