Iranian state media said 25 people, including civilians and members of the Islamic Revolutionary Guard Corps, had been killed. At least 60 others were wounded, including some who were in critical condition.
Iran’s state-run IRNA news agency said at least four gunmen — some apparently dressed in military uniforms — opened fire on reviewing stands for a parade to mark the start of Iran’s war with Iraq during the 1980s. The provincial governor in Ahvaz, Gholamreza Shariati, told IRNA that two gunmen were killed and two others arrested.
It was not immediately clear who was behind the attack.
The Islamic State asserted responsibility in a statement carried by the affiliated Amaq News Agency. Iran is predominantly Shiite Muslim and is at odds with Sunni extremist groups such as al-Qaeda and the Islamic State, which view Shiites as heretics and have attacked Shiite sites and gatherings across the region.
But a competing assertion of responsibility also came from the Arab Struggle Movement for the Liberation of Ahvaz, a separatist faction that seeks an Arab-led state in southwestern Iran.
Yacoub Hor al-Tostari, a spokesman for the separatist group, told the Associated Press that Ahvaz militants carried out the attack.
Islamic State militants were believed to be linked to twin strikes in June 2017 against Iran’s parliament and the shrine of the late Ayatollah Ruhollah Khomeini, leader of Iran’s 1979 Islamic revolution. At least 18 people were killed in the two attacks.
However, the Islamic State often asserts a role in bloodshed around the world without providing clear evidence to support the claims, and its statement about the Ahvaz attack was received skeptically. The statement initially said the attack had targeted Iranian President Hassan Rouhani, who was not at the parade.
Iranian Foreign Minister Mohammad Javad Zarif tweeted Saturday that Iran would “respond swiftly and decisively in defense of Iranian lives.”
“Terrorists recruited, trained, armed & paid by a foreign regime have attacked Ahvaz. Children and journos among casualties,” Zarif wrote. “Iran holds regional terror sponsors and their US masters accountable for such attacks.”
Although the identities of the attackers remained unclear, a spokesman for Iran’s Revolutionary Guard Corps blamed minority Arab separatists for the bloodshed. He accused Saudi Arabia, a regional rival of Iran, of funding the militant group he said was responsible. Earlier, state television said the attack was carried out by Sunni “takfiri terrorists,” a reference to Muslims viewed as traitors to their own faith.
In messages on Twitter, Iran’s supreme leader, Ayatollah Ali Khamenei, blamed the attack on regional allies of the United States, calling it “a continuation of conspiracies by US-backed regimes in region which have aimed at creating insecurity in our dear country.”
Ahvaz is home to the small Arab minority in Iran, a mainly ethnic Persian country, and the Ahvaz area has been the site of sporadic unrest. This summer, residents staged protests amid severe water shortages. Members of the Ahvaz Arab minority have long accused the government in Tehran of neglect.
In one video posted online, soldiers participating in the parade initially seem confused about the source and nature of the gunfire. They soon scatter as the shooting intensifies.
“Get on the ground!” one man yells. Images also showed bloodied soldiers limping toward ambulances and troops helping escort women and children to safety.
Hassan Shahvarpour, a regional commander of the Revolutionary Guard Corps in Khuzestan, the province where Ahvaz is located, said all four of the assailants were killed in clashes with security forces on the scene, state television reported.
A resident of Ahvaz, who spoke on the condition of anonymity, said that police and security forces were “everywhere” in the city and that movement between neighborhoods was difficult.
In Washington, National Security Council spokesman Garrett Marquis said: “The United States stands with the Iranian people and encourages the regime in Tehran to focus on keeping them safe at home.”
The assailants in the 2017 attacks were identified as members of Iran’s Sunni Kurdish minority who had joined the Islamic State. In July, Iran executed eight people convicted of involvement in those attacks.
Adam Taylor and Brian Murphy in Washington contributed to this report.
Correction: An earlier version of this report included an incorrect reference to the year of Iran’s Islamic revolution. It occurred in 1979.