The US Department of the Treasury has imposed sanctions against Iranian Foreign Minister Mohammad Javad Zarif.
The sanctions freeze any assets Mr Zarif may have in America, the department said.
"Javad Zarif implements the reckless agenda of Iran's Supreme Leader (Ayatollah Ali Khamenei)," Treasury Secretary Steven Mnuchin said.
Mr Zarif tweeted the US had imposed sanctions on him because it considered him as a threat to its agenda.
Tensions between the US and Iran have heightened since the US last year withdrew from the 2015 nuclear deal that aimed to curb Iranian nuclear activities.
There are growing concerns that a number of recent incidents in the Gulf could lead to a military conflict in the vital shipping region.
On Wednesday, the US extended waivers which allow Russia, China and European countries to continue civilian nuclear cooperation with Iran.
White House security adviser John Bolton said on Wednesday that it was a "short 90 day extension".
"I think the idea here is we are watching those nuclear activities very, very closely," he added.
What did the US say?
Mr Mnuchin described Mr Zarif as the Iranian "regime's primary spokesperson around the world".
"The United States is sending a clear message to the Iranian regime that its recent behaviour is completely unacceptable.
"At the same time 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," Mr Mnuchin said.
How did Mr Zarif respond?
Mr Zarif said the US move "has no effect on me or my family, as I have no property or interest outside of Iran".
The US' reason for designating me is that I am Iran's "primary spokesperson around the world"— Javad Zarif (@JZarif) July 31, 2019
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.
"The US reason for designating me is that I am Iran's 'primary spokesperson around the world. Is the truth really that painful?" he asked.
"Thank you for considering me such a huge threat to your agenda."
What about the 2015 nuclear deal?
Last year, the US unilaterally withdrew from the agreement between Iran and world powers.
Washington has since reimposed tight sanctions affecting the Iranian economy, and also
The other parties of the 2015 deal - China, France, Germany Russia and the UK - criticised Mr Trump's decision and said they remained fully committed to the deal.
Iran responded by breaching the limit on its stockpile of low-enriched uranium set under the nuclear deal.
Tehran stepped up production of enriched uranium, used to make reactor fuel but also potentially nuclear bombs, in May.
Last week, talks were held in Vienna to try to save the nuclear deal.
After meeting representatives from Britain, France, Germany, Russia and China, a senior Iranian official said the atmosphere had been "constructive".
And what about recent tanker incidents?
Tensions between the UK and Iran rose in July after British forces seized the Iranian tanker, Grace 1, off Gibraltar. It was alleged to be carrying oil to Syria in breach of EU sanctions, a claim denied by Iran.
Several days later the British-flagged Stena Impero was impounded by Iran, which said it had been "violating international maritime rules".
Britain sent a second warship on Sunday to escort its ships sailing through the Strait of Hormuz.
Recent incidents have also included:
- US claims that an Iranian drone was destroyed after coming close to the USS Boxer aircraft carrier earlier this month
- A US threat to carry out air strikes after Iran shot down a US drone in June
- Explosions that damaged two tankers near the Strait of Hormuz, also in June
- Blasts that hit four tankers in the UAE's territorial waters in May