At least 25 people have been killed and 53 wounded in an attack on a military parade in southwestern Iran, the country's official IRNA news agency reported, prompting President Hassan Rohani to pledge a "crushing" response to threats against the country.
Citing "knowledgeable sources," IRNA said gunmen dressed in military uniforms opened fire at the event in the southwestern city of Ahvaz on September 22, targeting a stand where Iranian officials were gathered to watch an annual event marking the start of the country’s 1980-88 war with Iraq.
"There are a number of nonmilitary victims, including women and children who had come to watch the parade," IRNA quoted an unnamed official source as saying.
Iran's foreign minister blamed "terrorists paid by a foreign regime" for the attack.
The Islamic State extremist group and an anti-government umbrella group, Ahvaz National Resistance, both claimed responsibility for the attack.
Rohani said in a statement that "the response of the Islamic Republic of Iran to the smallest threat will be crushing."
"Those who give intelligence and propaganda support to these terrorists must answer for it," he said.
Iran's Supreme Leader Ayatollah Ali Khamenei claimed the attack was linked to the United States and "allies in the region" but provided no details supporting that allegation.
"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, without naming any other countries.
The IRNA report, which said the death toll was expected to rise, did not say whether members of Iran's elite Islamic Revolutionary Guards Corps (IRGC) were among the fatalities.
But the semiofficial Tasnim news agency said at least eight IRGC members had been killed.
"Three of the terrorists were killed on the spot and a fourth one who was injured died in hospital," Brigadier General Abolfazl Shekarchi, a senior spokesman for Iran's armed forces, told state television.
IRNA earlier reported that two alleged attackers had been detained.
A video distributed to Iranian media showed soldiers crawling on the ground as gunfire rang out.
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 "U.S. masters" for the attack, adding that Iran would respond "swiftly and decisively."
Yaghub Hur Totsari, spokesman for Ahvaz National Resistance, an umbrella group that claims to defend the rights of the Arab minority in Khuzestan Province, told RFE/RL's Radio Farda that the group was responsible for the attack.
Islamic State's Amaq agency also said it carried out the attack, but provided no evidence to back the claim.
The semiofficial ISNA news agency said an unnamed spokesman for the IRGC had 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.
Russian President Vladimir Putin expressed condolences to Iran after the attack, saying Moscow was ready to boost joint efforts in the fight against terrorism, RIA Novosti quoted the Kremlin as saying.
Ahvaz is the capital of Iran's oil-rich Khuzestan Province. The province in the past has seen Arab separatists attack oil pipelines.
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.
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.
With reporting by AP, AFP, Reuters, and dpa