A shooting at a military parade on Saturday killed at least 29 people in the southwestern city of Ahvaz in Iran.
The nation’s President, Hassan Rouhani, vowed Tehran’s response to the deadliest attack on Iranian soil for years would be “crushing” while implying the US had a part to play.
But US envoy to the UN, Nikki Haley, has implored Iran to “look in the mirror” for causes of the attack before it blames the US.
Responding to Mr Rouhani’s outspoken censure of the US, in which he accused it of facilitating the attack, Ms Haley told CNN: “He’s got the Iranian people protesting, every ounce of money that goes into Iran goes into his military, he has oppressed his people for a long time and 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 in the mirror.”
Two groups have claimed responsibility for the Ahvaz attack — but neither has provided any evidence.
The anti-government Arab group — Ahvaz National Resistance — and ISIS both said they had committed the brutal killings.
On Sunday, before a flight to a UN session in New York, Mr Rouhani pledged Iran would “not let this crime stand”.
He said: “It is absolutely clear to us who committed this crime and whom they are linked to.
“The small puppet countries in the region are backed by America, and the US is provoking them and giving them the necessary capabilities.”
Although Mr Rouhani did not mention specifically who the so-called “puppet” nations were, he apparently accused US-backed nations of being complicit in the atrocities.
Mr Rouhani alleged a Gulf state had supported the “financial weaponry and political needs” of the perpetrators.
The comments appear to allude to the alleged involvement of Saudi Arabia, the UAE and Bahrain.
Both Iran and Saudi Arabia have been fighting for religious and political dominance for decades.
The nations are embroiled in proxy wars in Middle Eastern regions, backing rival factions in Yemen and Syria.
The decades-old feud is heightened by religious differences — Iran is largely Shia Muslim, while Saudi Arabia sees itself as the leading Sunni Muslim power.
Meanwhile, relations between the US and Iran continue to spiral downwards following US President Donald Trump’s withdrawal from the historic nuclear deal struck between Iran, China, France, the UK, the US and Germany.
The deal, known as the Joint Comprehensive Plan of Action, was intended to curb Iran’s nuclear energy programme, which the West maintained was to cover up the development of a nuclear bomb.
But Trump withdrew from the deal, branding it “horrible”, “laughable” and “disastrous”.
The US has also slapped on punitive sanction on Iran in response to the detention of US pastor Andrew Brunson.
The UK is desperately trying to find innovative ways to circumvent the sanctions, which restrict nations from trading with Iran — one of the world’s largest crude oil producers.