The president of Iran has labelled the US a "bully" after an attack on a military parade where at least 29 people were killed.
Hassan Rouhani has accused America of looking to create insecurity in the Islamic Republic, claiming the perpetrators were helped by the US.
Four gunmen opened fire on the crowd on Saturday, killing civilians and 12 members of the country's elite Revolutionary Guards.
Speaking before leaving Tehran to attend the UN General Assembly in New York, Mr Rouhani accused US-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 Mr Rouhani.
He warned the United States will regret its "aggressiveness".
The US responded on Sunday, condemning the atrocity and rejecting any allegation the US would be complicit in a terror attack.
"The United States condemns any terrorist attack anywhere, period. We've always stood by that," Nikki Haley, the US ambassador to the UN, told CNN.
"He needs to look at his own base to figure out where that's coming from. He can blame us all he wants. The thing he's got to do is look at the mirror," she continued.
On Sunday, Iran summoned diplomats from Britain, Denmark and the Netherlands over allegedly harbouring "members of the terrorist group" that launched an attack on a military parade in the country's southwest.
Iranian armed forces spokesman Brigadier General Abolfazl Shekarchi earlier said the dead included a young girl and a former serviceman in a wheelchair.
"Of the four terrorists, three were sent to hell at the scene, while the fourth who had been wounded and arrested went to hell moments ago due to his severe wounds," Mr Shekarchi told state television.
It reported 29 dead and 57 wounded in the southwestern city of Ahvaz.
Both Islamic State and an Iranian ethnic Arab opposition movement called the Ahvaz National Resistance have claimed responsibility for the attack.
On Sunday evening, Islamic State's Amaq agency posted video of three men in uniform, claiming they were about to carry out the attack.
"The response of the Islamic Republic of Iran to the smallest threat will be crushing", Mr Rouhani said on his official website. "Those who give intelligence and propaganda support to these terrorists must answer for it."
Shortly after the attack, which began half an hour after the parade started at 8.30am local time (6am in the UK), foreign minister Mohammed Javad Zarif blamed the attack on "regional terror sponsors" and their "US masters".
"Iran will respond swiftly and decisively in defence of Iranian lives," he wrote on Twitter.
The attack was one of the worst ever against the Guards, who are the most powerful and notorious military force in the country.
The shooting lasted for about 10 minutes.
Footage in a video being shared on social media showed the crowd dropping to the floor as the rattle of gunfire was heard in the background.
A second video capturing the aftermath of the attack showed a chaotic scene of people fleeing and paramedics trying to help the injured.
Photos shared by a state news agency showed a man in military fatigues with blood running down his back being helped towards a car by a group of men.
A picture of a bloodied rucksack was also shared.
The parade in the southwestern city was being held to mark the anniversary of the start of the eight-year war with Saddam Hussein's Iraq in 1980.
IS attacked Iran's parliament and the shrine of Ayatollah Ruhollah Khomeini in Tehran on 7 June 2017. At least 18 people were killed and more than 50 wounded in the attack.
The assault shocked Tehran, which largely has avoided militant attacks in the decades after the 1979 Islamic Revolution, which saw Khomeini topple the Western-backed shah to become Iran's first supreme leader until his death in 1989.