Iran summoned diplomats from Britain, Denmark, and the Netherlands early Sunday for allegedly harbouring "members of the terrorist group" that launched an attack in the southwest, killing at least 25 people and wounding over 60.
Women and children scattered along with once-marching Revolutionary Guard soldiers as militants disguised as soldiers opened fire on an annual Iranian military parade in Ahvaz on Saturday, the chaos captured live on state television.
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.
President Hassan Rouhani ordered Iran's Intelligence Ministry to immediately investigate the attack.
Speaking before leaving Tehran on Sunday to attend the U.N. General Assembly in New York, Rouhani accused the United States of being a "bully" that wants to create insecurity in the Islamic Republic.
Rouhani also accused U.S.-backed Gulf Arab states of providing financial and military support for anti-government ethnic Arab groups.
"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 Rouhani.
Iran's Supreme Leader Ayatollah Ali Khamenei said the attack was linked to US-backed Gulf states attempting to bring insecurity to the region.
"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, stressing the need to bringing the perpetrators to justice.
The victims included women and children who had come to watch the parade.
Khamenei did not name the states, but US allies in the region include Iran's arch-foe Israel and also the Arab Gulf states, especially Saudi Arabia.
The attack started at 09:00 local time on Saturday and appeared to involve four gunmen.
State television said the attack targeted a stand where Iranian officials had gathered to watch an annual event marking the start of the Islamic Republic’s 1980-88 war with Iraq.
The assailants fired at civilians and attempted to attack military officials on the podium.
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.
Two of the gunmen were killed by security forces, while the other two were arrested, according to the local deputy governor, Ali Hosein Hoseinzadeh.
“Shooting began by several gunmen from behind the stand during the parade. There are several killed and injured,” a correspondent told state television.
The semi-official news agency Mehr said further shooting broke out as some of the attackers who managed to escape were being chased.
A video on state television’s website showed confused soldiers at the scene of the attack.
Iran was holding similar parades in several cities including the capital Tehran and the port of Bandar Abbas on the Gulf.
Tensions between Iran and Saudi Arabia have surged 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.
A year ago in the first attack claimed by Islamic State in Tehran, 18 people were killed at the parliament and mausoleum of Ayatollah Ruhollah Khomeini, the founder and first supreme leader of the Islamic Republic.