The US imposed sanctions on Iranian Foreign Minister Javad Zarif on Wednesday, reducing the likelihood of a diplomatic solution to tension that has nearly driven Washington and Tehran to war.
The sanctions prohibit Mr Zarif from making transactions to or through the US. He was designated for acting on behalf of Iran's supreme leader, Ayatollah Ali Khamenei, who was sanctioned in June.
“Javad Zarif implements the reckless agenda of Iran’s supreme leader and is the regime’s primary spokesperson around the world," Steve Mnuchin, the US Treasury Secretary, said in a statement.
"The United States is sending a clear message to the Iranian regime that its recent behaviour is completely unacceptable."
Iran's President Hassan Rouhani responded on Thursday by saying it showed Washington was "afraid" of Mr Zarif.
"They are afraid of our foreign minister's interviews," Mr Rouhani said in a televised speech, referring to recent interviews Mr Zarif gave to media in New York.
"It is completely clear that the foundations of the White House have been shaken by the words and logic of an informed, devoted and diplomatic individual.
"They are doing childish things now," he continued.
The administration of US President Donald Trump announced last month that it planned to sanction Mr Zarif.
Some Republicans were quick to rally around Mr Trump's decision to sanction the minister.
"I fully support the Trump administration’s decision to sanction Iranian Foreign Minister Javad Zarif and designate him a supporter of terrorism – because he is," Senator Lindsey Graham said on Twitter:
Mr Graham then called Mr Zarif "the face and voice of the leading state-sponsor of terrorism on the planet".
The Iranian diplomat played an instrumental role in negotiating the 2015 nuclear deal from which the Trump administration withdrew in 2017.
“We do not consider him to be our primary point of contact,” a US official said.
The official accused Mr Zarif of spreading the regime’s propaganda.
Asked whether Mr Zarif, who recently visited New York for a UN session, would be allowed to travel to the US, the official said Washington would treat his visa applications with “a high degree of scepticism.
The official said those attending the UN General Assembly meetings in September would be considered on a case-by-case basis.
The order warns that “persons who engage in certain transactions with the individual designated today may themselves be exposed to designation".
Mr Zarif is known to engage with the policy and experts community in the US.
“At the same time as the Iranian regime denies Iranian citizens’ access to social media, Foreign Minister Javad Zarif spreads the regime’s propaganda and disinformation around the world through these mediums,” the sanctions statement read.
Any of his property or interests in property in the US would be blocked or reported.
Reacting to the sanctions, Mr Zarif tweeted: “The US reason for designating me is that I am Iran's ‘primary spokesperson around the world’.
"Is the truth really that painful? It has no effect on me or my family, as I have no property or interests outside of Iran. Thank you for considering me such a huge threat to your agenda."
US Secretary of State Mike Pompeo said: "Iran’s Foreign Ministry is not merely the diplomatic arm but also a means of advancing many of the supreme leader’s destabilising policies.”
He called Mr Zarif "a key enabler of Ayatollah Khamenei’s policies throughout the region and around the world”, and said the designation reflected this.
Henry Rome of the Eurasia Group said the new sanctions “will likely not have a meaningful effect on Mr Zarif’s bottom line or his ability to conduct diplomacy, but they will erode the prospect of US-Iran negotiations this year".
"They may cause headaches on Mr Zarif’s foreign trips, such as with airplane refuelling, but host governments will likely find ways around compliance concerns," Mr Rome said.
He said it was possible the US would seek to restrict Mr Zarif’s entry for the UN General Assembly in September.
Mr Rome said the timing, 37 days after threatening such action, was related to the Trump administration's decision to renew waivers for civil nuclear co-operation with Iran for another 90 days, over the objections of the State Department, the National Security Council and Republican hawks in Congress.
Designating Mr Zarif would distract from the waivers, he said.
Mr Zarif's primary place of diplomacy in the US is at the Iranian mission to the UN in New York, and the UN headquarters.
Speaking there two weeks ago he mainly seemed amused at the prospect of being sanctioned.
After the US decision on Wednesday, senior officials at the UN said the move was unhelpful but probably more symbolic than anything else.
"We do not support this American policy of maximum pressure," a top European diplomat said. "We're not going to join in and we've told the US that in our meetings with them."
Asked if the designation could harm efforts to maintain the nuclear deal, the diplomat said: "The restrictions on Iranians here are already very tough. We shall have to see if it stops the foreign minister from coming here."
Iranian diplomats at the UN, along with those of Cuba, Syria and North Korea, and their families have long been restricted to a 40-kilometre radius from Columbus Circle in Midtown Manhattan.
The US earlier in July told the Iranian mission that the rules would be further tightened, meaning they may only travel between the UN, the Iranian UN mission, the Iranian UN ambassador’s residence and John F Kennedy airport.