ISIS has claimed responsibility for a massacre at a military parade in Iran which claimed 24 lives.
Four gunmen, who were also killed, opened fire on Revolutionary Guards, injuring a further 60 troops in an attack captured in horrifying video.
The terror group’s Amaq news agency said so-called Islamic State supporters were behind the killings in the city of Ahvaz.
But another group, ethnic Arab opposition movement the Ahvaz National Resistance, has also said it carried out the attack.
Neither group has provided any evidence of their involvement.
An injured child is carried to safety following the deadly attack
Soldiers lying on the ground as gunmen open fire on the parade in Ahvaz, Iran
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Officials at the scene have blamed Israeli intelligence service Mossad and the US.
Officials had gathered to watch an annual event commemorating the start of the Islamic Republic’s 1980-88 war with Iraq when gunshots rang out.
Women and children also died in the violence, state news IRNA agency quoted an unnamed official source as saying.
The Islamic Revolutionary Guards Corp (IRGC) have defended Shi’ite clerical rule since Iran’s 1979 Islamic Revolution.
The Guards also play a major role in Iran’s regional interests in countries such as Iraq, Syria and Yemen.
The attack happened on a military parade in Ahvaz
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A video distributed to Iranian media showed soldiers crawling on the ground as gunfire blazed in their direction.
One soldier picked up a gun and got to his feet as women and children fled for their lives.
A video on state television’s website showed confused soldiers at the scene of the attack.
Standing in front of the stand, one asked: "Where did they come from?" Another responded: "From behind us."
The bloodshed struck a blow to security in OPEC oil producer Iran, which has been relatively stable compared with neighbouring Arab countries that have grappled with upheaval since the 2011 uprisings across the Middle East.
Injured soliders lying on the ground after gunmen opened fire on the parade
State television blamed "takfiri elements", a reference to Sunni Muslim militants, for the attack. Ahvaz is in the centre of Khuzestan province, where there have been sporadic protests by minority Arabs in predominantly Shi’ite Iran.
An Iranian military spokesman said the gunmen were trained by two Gulf Arab states and had ties to the United States and Israel.
"These terrorists… were trained and organised 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."
Iran has suggested in recent weeks that it could take military action in the Gulf to block other countries’ oil exports in retaliation for U.S. sanctions intended to halt its sales of crude.