Top Trump administration officials pushed back hard Sunday against Iranian claims that the USA and its allies were behind a deadly attack on a military parade over the weekend, saying that Tehran is fully responsible for fomenting a climate of unrest and instability inside its own borders and across the Middle East.
"It is absolutely clear to us who has done this, which group it is and to whom they are affiliated", Rouhani said on state television ahead of his departure from Tehran for this week's U.N. General Assembly in NY.
"They are not from Daesh [ISIS] or other groups fighting [Iran's] Islamic system", Brig Gen Abolfazl Shekarchi told the official news agency IRNA.
The parade in Ahvaz, a city with a population of more than a million people, was one of many happening nationwide in honor of Sacred Defense Week, marking the 38th anniversary of Iran's eight-year war with Iraq.
Iranian government and military officials have pointed the finger at Gulf states, the U.S. and Israel, with all of whom Iran has longstanding tensions.
Mr Khamenei did not name the "regional states" that he believed were behind the attack.
"Iran's answer (to this attack) is forthcoming within the framework of law and our national interests", said Rouhani, adding that the United States would regret its "aggressiveness". "Those who give intelligence and propaganda support to these terrorists must answer for it".
The official state news agency IRNA said 53 people had also been wounded and that many were in a critical condition.
"They are fed by saudi Arabia, and they have tried to overshadow the power of the armed forces", the iranian, he added.
An Iranian soldier carrying a child to safety
"Several gunmen began shooting from behind the stand during the parade".
Foreign Ministry also summoned UK's charge d'affaires, in absence of its ambassador, and protested over live broadcasting of an interview with spokesman of the terrorist group behind the attack at "Iran International" channel.
Syria is a major ally of Iran, benefiting from generous military and financial aid from Tehran to fight the seven-year-old armed rebellion against the Syrian government.
Attacks on the military are rare in Iran.
It was one of the worst attacks against the Islamic Revolutionary Guards Corp, the sword and shield of Shi'ite clerical rule in Iran since its 1979 Islamic Revolution.
The attack, which took place around 09: 00 (05: 30 GMT), according to the agency semi-official Isna, was conducted in Ahvaz, the capital of the province of Khuzestan, populated mostly by Arabs.
Iran's President Hassan Rouhani claimed Sunday that an unnamed US-allied Arab country in the region supported the group that attacked the parade.
There was no immediate Gulf Arab comment on his remarks.