Iranian President Hassan Rouhani has 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.
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 they were 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.
"The small puppet countries in the region are backed by America, and the US is provoking them and giving them the necessary capabilities."
He added that Iran would voice its views on "the global situation" at the UN, adding: "The US showed the world their bullying nature and they keep continuing their unilateral policies."
The two sides have had thorny relations for decades.
The US has accused Iran of running a clandestine nuclear weapons programme, which Iran denies.