Gunmen attacked a military parade in the southwestern Iranian city of Ahvaz on Saturday, killing 24 and wounding 53, Iran's state-run IRNA news agency said.
An unnamed spokesman for the Revolutionary Guards blamed Arab nationalists, who he said were backed by Saudi Arabia, the semi-official ISNA news agency reported.
A video on state television's website showed confused soldiers at the scene of the attack.
"Several gunmen began shooting from behind the stand during the parade".
A general view of the attack during the military parade in Ahvaz, Iran September 22, 2018.
There was no immediate claim of responsibility for the attack in Iran's southwest.
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State television aired footage of the aftermath of the assault on Ahvaz's Quds, or Jerusalem, Boulevard, which like many other places around the country saw an annual parade marking the start of Iran's long 1980s war with Iraq.
The IRNA report said those wounded in the attack Saturday included a woman and a child.
Zarif added that the gunmen were "terrorists recruited, trained, armed & paid by a foreign regime". However, Arab separatist groups in the region have launched attacks on oil pipelines there in the past. The Guard also has vast holdings in Iran's economy. Some media reports said that there were four assailants and that at least two had been killed. The province has a large ethnic Arab community, many of them Sunni, the Muslim denomination to which IS adheres and which is a minority in Shiite Iran.
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.
Last year, in the first deadly assault claimed by Islamic State in Tehran, 18 people were killed at the parliament and mausoleum of Ayatollah Ruhollah Khomeini, the founder and first supreme leader of the Islamic Republic.