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Militants disguised as soldiers opened fire on an annual Iranian military parade in the country's oil-rich southwest, killing at least 25 people and wounding over 60 in the deadliest terror attack to strike the country in nearly a decade.

Women and children scattered along with once-marching Revolutionary Guard soldiers as heavy gunfire rang out Saturday at the parade in Ahvaz, the chaos captured live on state television.

The region's Arab separatists, once only known for nighttime attacks on unguarded oil pipelines, claimed responsibility for the brazen assault and Iranian officials appeared to believe the claim. Iran summoned diplomats from Britain, Denmark and the Netherlands early Sunday for allegedly harboring "members of the terrorist group" that launched the attack.

Iranian Foreign Minister Mohammad Javad Zarif blamed regional countries and their "U.S. masters" for funding and arming the separatists, issuing a stark warning as regional tensions remain high in the wake of the U.S. withdraw from the Iranian nuclear deal.

"Iran will respond swiftly and decisively in defense of Iranian lives," Zarif wrote on Twitter.

The attack came as rows of Revolutionary Guardsmen marched down Ahvaz's Quds, or Jerusalem, Boulevard. It was one of many around the country marking the start of Iran's long 1980s war with Iraq, commemorations known as the "Sacred Defense Week."

Journalists and onlookers turned to look toward the first shots, then the rows of marchers broke as soldiers and civilians sought cover under sustained gunfire. Iranian soldiers used their bodies at time to shield civilians in the melee, with one Guardsman in full dress uniform and sash carrying away a bloodied boy.

"Oh God! Go, go, go! Lie down! Lie down!" one man screamed as a woman fled with her baby.

In the aftermath, paramedics tended to the wounded as soldiers, some bloodied, helped their comrades to ambulances. Video obtained by The Associated Press of the aftermath showed bodies of soldiers, some appearing lifeless, laying on the ground in pools of blood. One had a blanket covering him. A man screamed in grief.

The attack killed at least 25 people and wounded over 60, according to the state-run IRNA news agency. It said gunmen wore military uniforms and targeted a riser where military and police commanders were sitting. At least eight of the dead served in the Revolutionary Guard, an elite paramilitary unit that answers only to Iran's supreme leader, according to the semi-official Tasnim news agency.

"We suddenly realized that some armed people wearing fake military outfits started attacking the comrades from behind (the stage) and then opened fire on women and children," an unnamed wounded soldier told state TV. "They were just aimlessly shooting around and did not have a specific target."

State TV hours later reported that all four gunmen had been killed, with three dying during the attack and one later succumbing to his wounds at a hospital.

President Hassan Rouhani ordered Iran's Intelligence Ministry to immediately investigate the attack.

"The president stressed that the response of the Islamic Republic of Iran to the slightest threat would be harsh, but those who support the terrorists should be accountable," IRNA reported.

Meanwhile, Supreme Leader Ayatollah Ali Khamenei described the attack as exposing "the atrocity and viciousness of the enemies of the Iranian nation."

"Their crime is a continuation of the conspiracies by the U.S.-backed regimes in the region which have aimed at creating insecurity in our dear country," Khamenei said in a statement. "However, to their dismay, the Iranian nation will persist on the noble and prideful path they have taken and will — like before — overcome all animosities."

Tensions have been on the rise between Iran and the U.S. The Trump administration in May pulled out of the 2015 nuclear accord with Iran, and since then has re-imposed sanctions that were eased under the deal. It also has steadily ramped up pressure on Iran to try to get it to stop what Washington calls "malign activities" in the region.

Despite that, the U.S. government strongly condemned the attack and expressed its sympathy, saying that "the United States condemns all acts of terrorism and the loss of any innocent lives."

Initially, authorities described the assailants as "takfiri gunmen," a term previously used to describe the Islamic State group. Iran has been deeply involved in the fight against IS in Iraq and has aided embattled Syrian President Bashar Assad in his country's long war.

But later, state media and government officials seemed to come to the consensus that Arab separatists in the region were responsible. The separatists accuse Iran's Persian-dominated government of discriminating against its ethnic Arab minority, though an Ahvazi Arab, Gen. Ali Shamkhani, serves as the secretary of Iran's Supreme National Security Council.

Khuzestan province also has seen recent protests over Iran's nationwide drought, as well as economic protests.

Iran has blamed its Mideast archrival, the Sunni kingdom of Saudi Arabia, for funding Arab separatists' activity. State media in Saudi Arabia did not immediately acknowledge the attack, though a Saudi-linked, Farsi-language satellite channel based in the United Kingdom immediately carried an interview with an Ahvazi activist claiming Saturday's attack.

Hamid Baeidinejad, Iran's ambassador to the U.K., called the channel's decision a "heinous act" in a post on Twitter and said his country would file a complaint with British authorities over the broadcast. Early Sunday, a Foreign Ministry statement quoting spokesman Bahram Qasemi similarly criticized Britain and said Danish and Dutch diplomats were told Iran "already warned" their governments about harboring Arab separatists.

Yacoub Hor al-Tostari, a spokesman for the Arab Struggle Movement to Liberate Ahvaz, told the AP that members of an umbrella group of Ahvazi activists his organization leads carried out the attack.

The attack undermined the Iranian government "on the day it wants to give a message to the world that it is powerful and in control," al-Tostari said. To bolster his claim, he gave details about one of the attackers that the AP could not immediately verify.

The Islamic State group also claimed responsibility for the attack in a message on its Amaaq news agency, but provided no evidence it carried out the assault. They also initially wrongly said the Ahvaz attack targeted Rouhani, who was in Tehran. The militants have made a string of false claims in the wake of major defeats in Iraq and Syria.

In Tehran, Rouhani watched a military parade that included ballistic missiles capable of reaching Israel and U.S. military bases in the Mideast. Rouhani said the U.S. withdraw from the nuclear deal was an attempt to get Iran to give up its military arsenal. United Nations inspectors say Iran is still complying with the deal, which saw it limit its nuclear program in exchange for the lifting of economic sanctions.

"Iran neither put its defensive arms aside nor lessens its defensive capabilities," Rouhani said. "Iran will add to its defensive power day by day."

Meanwhile, Iranian Gen. Abolfazl Shekarchi, a spokesman for the armed forces, alleged without evidence that the four militants involved in Saturday's attack "were dependent to the intelligence services of the U.S. and the Mossad" of Israel.

"They have been trained and organized in two Persian Gulf countries," he said, without elaborating.

Saturday's attack comes after a coordinated June 7, 2017 Islamic State group assault on parliament and the shrine of Ayatollah Ruhollah Khomeini, the leader of Iran's 1979 Islamic Revolution. At least 18 people were killed and more than 50 wounded.

That assault shocked Tehran, which largely has avoided militant attacks in the decades after the tumult surrounding the revolution.

In the last decade, mass-casualty militant attacks have been incredibly rare. In 2009, more than 40 people, including six Guard commanders, were killed in a suicide attack by Sunni extremists in Iran's Sistan and Baluchistan province.

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Gambrell reported from in Dubai, United Arab Emirates. Associated Press writers Sarah El Deeb in Beirut, Maamoun Youssef in Cairo and television producer Mohammad Nasiri in Tehran contributed to this report.