The U.S. State Department spokesman Mark Toner informed on the fact that Russia uses Iranian airbase for Syria airstrikes, added this use of the Iranian airbase is “unfortunate, but not surprising or unexpected”. Thus, such a behavior of Russian warplanes could qualify as a violation of UN Security Council Resolution 2231.
According to the U.S., Russia violates the Resolution of UNSG 2231, which stipulates prohibits the supply, sale and transfer of combat aircraft to Iran unless the council approves. The UN Security Council gave no such approval, so using by Russia of the Iranian base for airstrikes in Syria is very controversial activity, said Toner.
The Russian defense ministry, however, reported its TU-22M3 and SU-34 jets took flight from Hamedan Airbase in western Iran and hit targets in Syria. Prior to Iranian Hamedan Airbase, Russian air forces used Syrian Khmeimim Airbase.
Tehran and Moscow have recently signed the agreement, which reduces flight time by 60 percent, indeed. That should save Russian budget money and increase airstrike effectiveness at the same time.
In Syria, Moscow began its extensive air operations on September 30, 2015, caused criticism of international community.