Islamic State's Amaq agency has posted a video of three men in a vehicle who it said were on their way to carry out an attack on an Iranian military parade, which killed at least 25 and wounded 60 others.
A man wearing a baseball cap emblazoned with what appears to be a Revolutionary Guard logo discussed the impending attack in Farsi in the video. "We are Muslims, they are kafirs (non-believers)," the man says.
"We will destroy them with a strong and guerrilla-style attack, inshallah (God willing)," he adds.
Four assailants fired on a viewing stand in the southwestern city of Ahvaz on Saturday, where Iranian officials had gathered to watch an annual event marking the start of the Islamic Republic's 1980-88 war with Iraq. Soldiers crawled about as gunfire crackled. Women and children fled for their lives.
The assault, one of the worst ever against the most powerful force of the Islamic Republic, struck a blow at its security establishment at a time when the United States and its Gulf allies are working to isolate Tehran.
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Ahvaz National Resistance, an Iranian ethnic Arab opposition movement which seeks a separate state in oil-rich Khuzestan province, also claimed responsibility for the attack.
Nevertheless, a spokesman for the Iranian military said the attackers were trained by two Gulf states, and are connected to Israel and the U.S.
"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," Brigadier General Abolfazl Shekarchi told the official news agency IRNA.
There has been a blizzard of furious statements from top Iranian officials, including Iranian President Hassan Rohani, accusing Iran's adversaries the United States and Gulf states of provoking the bloodshed and threatening a tough response.
Nikki Haley, U.S. ambassador to the United Nations, rejected Rohani's accusations. "He's got the Iranian people ... protesting, every ounce of money that goes into Iran goes into his military, he has oppressed his people for a long time and he needs to look at his own base to figure out where that's coming from," she told CNN.