At least eight members of Iran's elite military have been killed with 20 people wounded in a gun attack during a parade in the southwestern city of Ahvaz, local media have said.
At first, those present appear to think that the gunfire could be part of the ceremony, but then quickly realize it is a hostile attack and immediately begin to scramble from the scene. Some images show bloodied soldiers in military fatigues, and ambulance sirens can be heard wailing in the background.
Semi-official news agency Mehr said further shooting broke out as some of the attackers who managed to escape were being chased.
At that point it marked the only attack by the Sunni extremists inside of Shia Iran, which has been deeply involved in the wars in Iraq and Syria, where the militants once held vast territory.
The fates of the gunmen were unclear.
No group immediately claimed responsibility for the assault in Ahvaz, however, Iran faced a bloody assault a year ago from the Islamic State group and Arab separatists in the region have attacked oil pipelines there in the past.
Iran holds "regional terror sponsors" and their "US masters" responsible for the attack, he said.
Ahvaz is the capital of Iran's oil-rich Khuzestan province, which has seen attacks on oil pipelines by Arab separatists.
Earlier reports described the assailants as "Takfiri", a term previously used to describe the Islamic State group.
The Revolutionary Guard is a paramilitary force answerable only to Supreme Leader Ayatollah Ali Khamenei.
Iran is also now facing intensified attacks in its Kurdish-majority provinces (Rojhilat) and along its Western border with the Kurdistan Region, where Kurdish opposition groups operate. At least 18 people were killed and more than 50 wounded in the attacks. The assault shocked Tehran, which has seen few militant attacks in the decades after the tumult surrounding the Islamic Revolution.