Several people were killed and up to 20 wounded when unknown gunmen opened fire during a military parade in the southwestern Iranian city of Ahvaz on Saturday, state television reported.
There was no immediate claim of responsibility.
The separatists accuse Iran's Shiite theocracy of discriminating against its Sunni Arab citizens.
Earlier, Governor-General of Ahvaz Gholamreza Shariati had told the state-run news agency IRNA that "the wounded have been transferred to Ahwaz hospitals".
Fars, another state news agency, says there were four gunmen and that the attack began half an hour after the parade began at 8.30am local time (6am in the UK).
"They are not from Daesh (Islamic State) or other groups fighting (Iran's) Islamic system. but they are linked to America and (Israel's intelligence agency) Mossad".
It has been reported that two of the assailents have been killed and two others arrested, Mehr reported.
Who carried out the assault also remained in question.
There has been no claim of responsibility for the attack in the city of Ahvaz.
The report Saturday described the assailants as "Takifiri gunmen", a term previously used to describe ISIS. Iran has been deeply involved in the fight against IS in Iraq and has aided embattled Syrian President Bashar Assad in his country's long war.
Yaghub Hur Totsari, spokesman for one of the two groups that identify themselves as the Arab Struggle Movement for the Liberation of Ahvaz, said the Ahvaz National Resistance, an umbrella organisation of all armed movements, was behind the attack but did not specify which group.
State media say Islamist militants are thought to be behind the attack.
Trump will fail in his confrontation with Iran, just like Iraq's Saddam Hussein, Iranian President Hassan Rouhani said, referring to the war in the 1980s between the two Middle Eastern powers and vowing that Tehran will not abandon its missiles.
At least 18 people were killed and over 50 wounded in the 2017 operation that saw gunmen carrying Kalashnikov rifles and explosives storm the Parliament complex. More than 53 people were injured in the attack. The Guards also play a major role in Iran's regional interests in countries such as Iraq, Syria and Yemen.
Iran's Revolutionary Guard is a paramilitary force answerable only to Supreme Leader Ayatollah Ali Khamenei. The assault shocked Tehran, which largely has avoided militant attacks in the decades after the tumult surrounding the Islamic Revolution.
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