The Revolutionary Guards said, according to the New York Times, the missile launches "targeted the headquarters and meeting place and suicide auto assembly line of ISIS terrorists" in the province of Deir al-Zour, where Islamic State group forces surround an estimated 200,000 people in a government-held section of the provincial capital of the same name.
While Iran has been fighting against ISIS in Syria for years, Iranian state media presented the launch of missiles as direct retaliation for ISIS terror attacks in the capital city of Tehran this month.
Iran's powerful Revolutionary Guard, a paramilitary force in charge of the country's missile program, said it launched six Zolfaghar ballistic missiles from the western provinces of Kermanshah and Kurdistan.
Early reports indicate full-fledged success as all Iranian missiles hit the predetermined targets; A large number of terrorists have been killed and their equipment and weapons destroyed.
Russia, which is flying its own air war in Syria to prop up President Bashar al-Assad, condemned the incident and said it had shut off a special "deconfliction" hotline with the USA military, though top officer General Joe Dunford later said the United States was seeking to re-establish the channel.
Senior Iranian officials, however, have pointed a finger at Saudi Arabia, Iran's Sunni regional rival.
Iran's Tasnim news agency said on Saturday that Saudi border guards had opened fire on an Iranian fishing boat in the Gulf on Friday, killing a fisherman. While Iran has other ballistic missiles it says can reach longer distances, Sunday night's launch appears to mark the longest strike it has launched overseas.
The launch of surface-to-surface medium range missiles by Iran's Revolutionary Guard into Syria's Deir ez-Zor province comes as Islamic State militants fleeing a USA -led coalition onslaught increasingly try to fortify their positions there. Israeli security officials said Monday they were studying the missile strike to see what they could learn about its accuracy and capabilities.
In last Wednesday's testimony to the House Foreign Affairs Committee, Tillerson accused Iran of seeking "hegemony" in the Middle East at the expense of United States allies like Saudi Arabia.
The attack also came as emboldened Sunni Arab states - backed by U.S. President Donald Trump - are hardening their stance against Iran.
He went on to add, "international and regional sponsors of terrorism need to understand the warning message of Iran's missile operations".
Activists say a "de-escalation agreement" brokered by Iran, Russia and Turkey in May has brought little relief. "We are tracking their actions and also their words".
"I think we're getting closer and closer to open conflict with Iran and Russian Federation, and the American public needs to know that we're moving very fast toward what could be another war inside the Middle East", Murphy told anchor Alisyn Camerota on CNN's "New Day", "something by the way that Donald Trump promised he wouldn't do when he ran for office".