Gunmen attacked a military parade in the southwest Iranian city of Ahvaz on Saturday, killing and wounding several people, state TV reported.
Fars news agency said the attack started at 09:00 local time (06:30 BST) and appeared to involve at least two gunmen.
State television showed images of the immediate aftermath.
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"The terrorists disguised as Islamic Revolution Guards Corps (IRGC) and Basiji (volunteer) forces opened fire to the authority and people from behind the stand during the parade", Governor of Khuzestan Gholam-Reza Shariati said, according to the state news agency IRNA.
No group immediately claimed responsibility for the assault in Ahvaz, which saw gunfire spray into a crowd of marching Guardsmen, bystanders and government officials watching from a nearby riser.
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Iranian state television quoted a spokesman for Iran's Revolutionary Guard Corps as saying that the attackers were affiliated with an Arab separatist group in the region. He said children and journalists were casualties in the attack.
He added that the gunmen were "terrorists recruited, trained, armed and paid by a foreign regime".
Ahvaz, in southwestern Khuzestan province, is home to the small Arab minority in Iran, which is mainly ethnic Persian. Some media reports said that there were four assailants and that at least two had been killed. However, those groups in the past previously have only attacked unguarded oil pipelines at night.
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Iran also suffered major attacks by IS extremists on June 7, 2017, when militants assaulted the parliament and the shrine of Ayatollah Ruhollah Khomeini in the capital, Tehran. The assault shocked Tehran, which has seen few militant attacks in the decades after the tumult surrounding the Islamic Revolution.