He also blamed the United States and its Gulf Arab allies for supporting destabilising forces in the Middle East.

The Iranian president did not name the country he suspected of orchestrating the attack but he could be referring to Saudi Arabia, the UAE or Bahrain.

All three are US allies and view Iran as a regional threat, due to the country’s support for a number of militant groups in the Middle East.

“All of those small mercenary countries that we see in this region are backed by America. It is Americans who instigate them and provide them with necessary means to commit these crimes,” Mr Rouhani said.

“America is acting like a bully towards the rest of the world ... and thinks it can act on the basis of brute force.”

“But our people will resist and the government is ready to confront America. We will overcome this situation [of sanctions] and America will regret choosing the wrong path.”

“The small puppet countries in the region are backed by America, and the United States is provoking them and giving them the necessary capabilities,” he added, speaking before leaving Tehran for New York, for the annual UN General Assembly.

Despite the Iranian president’s anger, US officials condemned the slaughter, saying that “the United States condemns all acts of terrorism and the loss of any innocent lives.”

Other countries and officials have also expressed their condolences.

Iranian soldiers, women, and children in Ahvaz on Saturday (EPA)

The Gulf Arab state of Qatar, which is at odds with both Saudi Arabia and the US, condemned the attack, as did UN Secretary General Antonio Gueterres, who issued a statement expressing sympathy with those caught up in the violence.

Syria, a major Iranian ally, did the same in a statement that declared that those sponsoring “terrorism in the region won’t be able to achieve their plots using these sordid crimes”.

An obscure Arab separatist group, Ahwazi Democratic Popular Front, has claimed responsibility for the incident.

They provided no evidence for the claim but Iranian officials believe they are responsible.

The group demands autonomy for Iran’s Arab minority, which has frequently complained of discrimination.

Isis later also claimed responsibility for the attack but offered no proof and provided incorrect information about the attack.

Iran has summoned diplomats from Britain, Denmark and the Netherlands over claims that their countries are allegedly harbouring “members of a terrorist group” behind the attack.

The Iranian Foreign Ministry also criticised Britain over a Saudi-linked, Farsi-language satellite channel which aired an interview with Ahwazi separatists immediately after the attack.

At least 70 people were wounded in the gunfire after four assailants began to shoot during the parade in Ahvaz, a city of 1.2 million people in Iran’s southwest which is the centre of the country’s petroleum industry.

The four gunmen were all killed during the annual parade, which marks Iran’s 1980-1988 war with Iraq.

Photos from the event show panicked civilians fleeing as shots ring out, in what was one of the worst attacks on Iran’s Revolutionary Guard.

The Guard issued a statement on Sunday vowing ”deadly and unforgiving revenge” on those responsible.