Iran's Supreme Leader Ali Khamenei, President Hassan Rouhani and other officials blamed unnamed "allies" of the United States for the attack on a military parade in the southwestern city of Ahvaz.
The death toll in the attack on Saturday, September 22 rose to 25, according to the Iranian state news agency IRNA. Almost half of those killed, 12 people were military from the Islamic Revolution Guard Corps (IRGC).
Officials say that the other 13 people were civilians, including a disabled war veteran and a four-year-old girl.
State television said the assault targeted a stand where Iranian officials were gathered to watch an annual event marking the start of the country’s 1980-88 war with Iraq.
An anti-government ethnic Arab group has taken responsibility in an interview with Radio Farda.
Yaghub Hur Totsari, spokesman for one of the two groups that identify themselves as the Arab Struggle Movement for the Liberation of Ahvaz, said the Ahvaz National Resistance, an umbrella organization of all armed movements takes repsonsibility.
He said that this was an act against "the oppression" of Arab-Iranians by government.
But Iran's top leader Khamenei said that the attack was linked to the United States' "allies in the region" and ordered security forces to bring the perpetrators to justice.
"This crime is a continuation of the plots of the regional states that are puppets of the United States, and their goal is to create insecurity in our dear country," Khamenei said in a statement published on his website.
He did not name the states, but U.S. allies in the region include Iran's arch-foe Israel and also the Arab Gulf states, especially Saudi Arabia.
Iran's President Hassan Rouhani vowed a "crushing response" to Saturday's attack on a military parade in the city of Ahvaz near the Iraqi border that left at least 29 people dead."
The response of the Islamic Republic of Iran to the smallest threat will be crushing", Rouhani said in a statement on his official website. "Those who give intelligence and propaganda support to these terrorists must answer for it."
An Iranian military spokesman claimed the gunmen were trained by two Gulf Arab states and had ties to the United States and Israel.
"These terrorists... were trained and organized by two ... Gulf countries," Brigadier General Abolfazl Shekarchi told the official news agency IRNA.
"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."
Iranian officials and media reported that four gunmen involved have been killed. Initially, it was reported that two attackers were being sought; meaning they had fled the scene.
Earlier reports described the attackers as “Takfiri gunmen,” a term used in the past to describe Sunni Muslim militants.
The semiofficial Fars news agency said two gunmen on a motorcycle wearing khaki uniforms carried out the attack.
Iranian Foreign Minister Mohammad Javad Zarif said on Twitter that the attackers were "terrorists recruited, trained, armed and paid by a foreign regime," but did not elaborate.
He blamed regional countries and their "US masters" for the attack, adding that Iran would respond "swiftly and decisively.
Ahvaz is the capital of Iran's oil-rich Khuzestan Province. The province in the past has seen Arab separatists attack oil pipelines.
ISNA said an unnamed spokesman for the elite Revolutionary Guards security force blamed Arab nationalists backed by Saudi Arabia for the attack.
Tensions between traditional rivals Iran and Saudi Arabia have increased in recent years, with the two countries supporting opposite sides in wars in Syria and Yemen and rival political parties in Iraq and Lebanon.
Attacks on the military are rare in Iran.
Last year, in the first deadly attack claimed by Islamic State in Tehran, 18 people were killed at the parliament and at the mausoleum of Ayatollah Ruhollah Khomeini, the founder and first leader of the Islamic Republic.