Iranian President Hassan Rouhani has fiercely criticised the US following a deadly attack on a military parade.
Gunmen opened fire at Revolutionary Guard troops in the south-western city of Ahvaz on Saturday, in an attack claimed by both an anti-government Arab group, and Islamic State militants.
Mr Rouhani said the "bully" US and the Gulf states it backed had enabled the attack.
The US has denied this and says it condemns "any terrorist attack".
Mr Rouhani will face Donald Trump at the UN General Assembly this week.
Saturday's attack killed 25 people, including 12 soldiers, civilians watching the parade, and a four-year-old girl.
Ahvaz National Resistance, an umbrella group that claims to defend the rights of the Arab minority in Iran's Khuzestan Province, said the group was behind the bloodshed, while IS also claimed the attack.
Neither group provided evidence to show it was involved.
Speaking on Sunday, before leaving for the UN in New York, Mr Rouhani vowed that Iran would "not let this crime stand".
"It is absolutely clear to us who committed this crime... and whom they are linked to," he said.
He alleged that a Gulf country had supported the "financial, weaponry and political needs" of the attackers.
He added: "The small puppet countries in the region are backed by America, and the US is provoking them and giving them the necessary capabilities."
"The US showed the world their bullying nature and they keep continuing their unilateral policies."
However, US ambassador to the UN Nikki Haley countered: "He [Rouhani] 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."
"The United States condemns any terrorist attack anywhere," she told CNN, but added: "He can blame us all he wants. The thing he's got to do is look at the mirror."
Mr Rouhani did not specify which "puppet" countries he was referring to - but his comments are widely thought to be directed at Iran's regional foe Saudi Arabia, and the UAE and Bahrain.
On Sunday, Iran's foreign ministry summoned a UAE envoy in protest at "biased statements" made in support of the attack.
An adviser to the UAE crown prince had tweeted that "attacking a military target is not a terrorist act" and "moving the battle deeper inside in Iran is a declared option".
Why does Iran blame its Gulf neighbours for the attack?
Iran has previously alleged that Saudi Arabia supports separatist activity among Iran's Arab minority.
Both countries have been struggling for regional political and religious dominance for decades, and are engaged in a number of proxy wars around the region, backing rival factions in Yemen and Syria.
The decades-old feud between them is exacerbated by religious differences - Iran is largely Shia Muslim, while Saudi Arabia sees itself as the leading Sunni Muslim power.
The UAE and Bahrain are closely allied with Saudi Arabia.
Iran has also alleged that the gunmen had links to its bitter foe Israel.
Iran considers Israel an illegitimate occupier of Muslim land - and has accused it of attempting to undermine the government.
Iran also summoned diplomats from the UK, the Netherlands, and Denmark on Saturday, accusing their countries of harbouring Iranian opposition groups.
And why are US-Iran relations so tense?
The two sides have had thorny relations for decades.
The US has accused Iran of running a clandestine nuclear weapons programme, which Iran denies.
In 2015, under then-president Barack Obama, the US and Iran reached a landmark nuclear deal - also signed by China, Russia, the UK, France and Germany - where Iran limited its nuclear activities in exchange for sanctions relief.
However, ties deteriorated after Mr Trump took office and then withdrew from the nuclear deal.
Since then, the US has brought sanctions back into place, despite opposition from EU partners, China and Russia, and warned that anyone trading with Iran would also be punished.
The US has also expressed support for anti-government protesters in Iran.
More generally, the US is deeply suspicious of Iranian activity in the Middle East - including its influence in Syria and Yemen - and is an ally of Israel and Saudi Arabia.
Mr Trump is chairing a UN Security Council meeting on Wednesday - and says the meeting will be focused on Iran.