U.S. and United Nations officials have condemned the attack on a military parade in southwestern Iran that state media said killed at least 25 people and wounded 60, an assault the country's supreme leader and the president blamed on "puppets" of the United States.
"The United States condemns all acts of terrorism and the loss of any innocent lives," State Department spokeswoman Heather Nauert said late on September 22.
"We stand with the Iranian people against the scourge of radical Islamic terrorism and express our sympathy to them at this terrible time," she added.
Earlier in the day, Iran's official IRNA news agency, citing "knowledgeable sources," reported that gunmen dressed in military uniforms opened fire at the event in the southwestern city of Ahvaz, 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.
A video distributed to Iranian media showed soldiers crawling on the ground as gunfire blazed in their direction.
Brigadier General Abolfazl Shekarchi, a senior spokesman for Iran's armed forces, told state television that "three of the terrorists were killed on the spot and a fourth one who was injured died in hospital."
The semiofficial Tasnim news agency said at least eight members of Iran's elite Islamic Revolutionary Guards Corps had been killed.
The Islamic State (IS) extremist group and an antigovernment Arab group, Ahvaz National Resistance, both claimed responsibility. Neither claim provided evidence.
Speaking before leaving Tehran to attend the UN General Assembly in New York, President Hassan Rohani accused U.S.-backed Gulf Arab states of providing "monetary, military, and political support" for antigovernment ethnic Arab groups, but provided no details supporting that allegation.
"The small puppet countries in the region are backed by America, and the United States is provoking them and giving them the necessary capabilities," said Rohani, who warned that the United States will "regret choosing the wrong path."
In a statement on September 22, Rohani pledged a "crushing" response to threats against the country, saying, "Those who give intelligence and propaganda support to these terrorists must answer for it."
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.
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."
United Nations Secretary-General Antonio Guterres issued a statement saying the world body "condemns the attack today in Iran's southwestern city of Ahvaz."
The statement said he "expresses his condolences to the families of the victims and to the Government and people of Iran. He wishes those injured a speedy recovery."
Russian President Vladimir Putin also expressed condolences to Iran after the attack, saying Moscow was ready to boost joint efforts in the fight against terrorism, the RIA Novosti news agency quoted the Kremlin as saying.
IRNA later reported that Tehran had summoned the envoys of Britain, the Netherlands, and Denmark, accusing them of harboring Iranian opposition groups.
"It is not acceptable that these groups are not listed as terrorist organizations by the European Union as long as they have not carried out a terrorist attack in Europe," Foreign Ministry spokesman Bahram Qasemi was quoted as saying.
Iranian opposition exile groups, including the Mujahedin-e Khalq Organization (MKO), in Western Europe and elsewhere have been active in protests against the government in Tehran since he 1979 Islamic Revolution in Iran.
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 that the group was responsible for the attack.
IS's Amaq agency also said it carried out the attack.
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.
Attacks on the military are rare in Iran.
Last year, in the first deadly attack claimed by IS in Tehran, 18 people were killed at the parliament and mausoleum of Ayatollah Ruhollah Khomeini, the founder and first leader of the Islamic Republic..