Iran's Supreme Leader Ayatollah Ali Khamenei on Monday accused the United States and Israel of involvement in the attack which killed 25 people at a military parade and said that Iran would "severely punish" those behind the bloodshed.The deputy head of Iran's Revolutionary Guards also said that the attackers had been paid by Saudi Arabia and the United Arab Emirates, and that they should expect a devastating response.
In Washington, US Defense Secretary Jim Mattis dismissed Iran's threat said it was "ludicrous" for Tehran to allege US involvement in the attack.
Asked whether Iran's threat gave him any concern, Mattis told reporters at the Pentagon: "No, it does not. We've been very clear that they shouldn't take us on like that. And I am hopeful that cooler, wiser heads will prevail."
In the southwestern city of Ahvaz, thousands packed the streets to mourn the victims of Saturday's assault, many chanting "Death to Israel and America." Twelve members of the elite Islamic Revolutionary Guard Corps were among the dead.
The coffins, wrapped in flags, were carried by mourners. Many held pictures of a four-year-old boy killed in the incident, one of the worst such attacks against Iran's most powerful military force.
Iran's Intelligence Ministry said some 22 people had been arrested in connection with the attack.
"Some 22 people involved in the attack in Ahvaz have been identified and arrested ... weapons, exclusive material and communication equipment were seized in the house that belonged to the five-member terrorist group that carried out the attack," a ministry statement said, according to the semi-official Tasnim news agency.
Gunmen fired on a viewing stand in Ahvaz where officials had gathered to watch an annual parade marking the start of Iran's 1980-88 war with Iraq.
Fars and IRNA news agencies said on Monday five attackers were killed, not four as previously reported by state media. The body of the fifth assailant had not been identified as it was mixed up with other casualties, Fars said.
"Based on reports, this cowardly act was done by people who the Americans come to help when they are trapped in Syria and Iraq, and are paid by Saudi Arabia and the UAE," Khamenei said on his website.
Guards Brigadier General Hossein Salami, in a speech broadcast on state TV, said: "You have seen our revenge before. You will see that our response will be crushing and devastating and you will regret what you have done."
Tasnim news agency quoted Salami as saying the "horrific crime" exposed the dark side of an alliance that the United States, Saudi Arabia and Israel had created to counter Iranian influence in the region.
The secretary of Iran's National Security Council said Tehran needed to talk to its neighbors to avoid tensions.
"It's essential to be fully aware and increase our constructive dialogs to neutralize the plots of enemies who want to create suspicion and disagreement among regional countries," Ali Shamkhani said.
He also said US sanctions against Iran were illegal and President Donald Trump was using them as a tool for "personal revenge."
US Secretary of State Mike Pompeo, asked by a Fox News interviewer if the United States played any role in the attack, said: "When you have a security incident at home, blaming others is an enormous mistake."