Senior officials in Tehran insisted the gunmen who opened fire on the ceremony in the southwestern city of Ahvaz were trained by two Gulf Arab states and had ties to Washington and the Israeli secret intelligence agency Mossad.
Brigadier General Abolfazl Shekarchi said: “These terrorists were trained and organised by two Gulf countries.”
“They are not from ISIS or other groups fighting Iran’s Islamic system.
“But they are linked to America and Mossad.”
Tehran foreign Minister Mohammed Javad Zarif said the assault was the handiwork of “regional terror sponsors”, language that usually refers to Iran’s enemies Saudi Arabia and Israel, and “their US masters”.
State television said the assault targeted a stand where Iranian officials had gathered to watch the parade.
At least 12 of the victims were members of Iran’s elite Revolutionary Guards who were taking part in n annual event marking the start of the Islamic Republic’s 1980-88 war with Iraq.
A security source said: “There are a number of non-military victims, including women and children who had come to watch the parade.”
The bloody attack comes as serious blow to Iran which has been relatively stable compared with neighbouring Arab countries that have grappled with upheaval since the 2011 uprisings across the Middle East.
There was no immediate claim of responsibility for the attack in the city of Ahvaz.
State television blamed Sunni Muslim militants for the attack. Ahvaz is in the centre of Khuzestan province, where there have been sporadic protests by the Arab minority in predominantly Shi’ite Iran.
Other security sources blamed Arab nationalists backed by Saudi Arabia for the attack.
The Revolutionary Guards are the most powerful and heavily armed military force in the Islamic Republic and also have a vast stake worth billions of dollars in the economy.
Kurdish militants killed 10 Revolutionary Guards in an attack on an IRGC post on the Iraqi border in July, Iran’s semi-official Tasnim news agency reported, the latest bloodshed in an area where armed Kurdish opposition groups are active.
Iran will be scrambling to determine the motives for the Saturday’s high-profile attack as it faces growing US pressure.
President Donald Trump decided in May to pull the United States out of the 2015 international nuclear deal with Tehran and reimpose sanctions in a bid to isolate the Islamic Republic.
Last year, in the first deadly assault 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.
A video on state television’s website showed confused soldiers at the scene of the horror.
Standing in front of the stand, one asked: “Where did they come from?” Another responded: “From behind us.”
Iran State News Agency said four militants carried out the attack and two of them were killed.
Tensions between mainly Shi’ite Iran and mostly Sunni 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.