According to the Russian news agency, Sputnik, Russia has been helpful in negotiations with Iran, Kirby added.
In April, Russian President Vladimir Putin lifted the ban on S-300 deliveries shortly after the P5+1 group of international negotiators and Iran reached a framework nuclear agreement.
Last month, Iran's chief nuclear negotiator Abbas Araqchi said that the S-300 air defense system is not subject to Security Council resolution.
Araqchi reiterated that the weapons that their sales to Iran would be subject to the restrictions are seven items and this would not include S-300.
“Purchasing the S-300 air defense system is out of the jurisdiction of the Security Council's recent resolution,” he added.