The United States on Wednesday imposed sanctions against Iran's foreign minister, Mohammad Javad Zarif, effectively slamming the door on the country's top diplomat.

The sanctions freeze any of Zarif's assets in the United States or that are controlled by US entities, the government announced, saying it also will curtail his international travel.

"Zarif implements the reckless agenda of Iran's Supreme Leader, and is the regime's primary spokesperson around the world. The United States is sending a clear message to the Iranian regime that its recent behavior is completely unacceptable," Treasury Secretary Steven Mnuchin said in a statement.

The designation of Zarif under the same sanctions earlier applied to Iran's supreme leader Ayatollah Ali Khamenei is the latest blow by President Donald Trump to the Obama administration's attempt to end nuclear tensions through negotiations.

But in a mixed message to Tehran, Washington on Wednesday extended waivers for three Iranian civil nuclear projects, to avoid upsetting the other signatories to the 2015 Iran nuclear agreement — China, Russia, Germany, France and Britain.

"This is a short 90 day extension," said White House National Security Advisor John Bolton, a champion of the hawkish policy towards Tehran.

"We are watching those nuclear activities very, very closely, they remain under daily scrutiny," he told Fox Business.

Zarif has been at the heart of complex talks with foreign capitals over Iran's nuclear power industry, which Tehran says is peaceful, but Washington and regional allies including Israel insist is cover for a secret weapons programme.

But a senior Trump administration official said that Zarif's diplomatic image — bolstered by his fluent English, self-effacing humour and background as a US-educated academic — was false.

"The key issue is that he has had this veneer [...] of being the sincere and reasonable interlocutor for the regime. Our point today is that he is no such thing," the official said on condition of anonymity.

"Today President Trump decided enough was enough," the official said, accusing Zarif of functioning as "propaganda minister, not foreign minister".

Zarif shot back, tweeting that Washington is trying to silence Iran on the international stage.

"The US' reason for designating me is that I am Iran's 'primary spokesperson around the world' Is the truth really that painful?" he tweeted.

Iranian President Hassan Rouhani said the sanctions on Zarif showed that the US was 'afraid' of the top diplomat.

Short leash at UN

In addition to attempting to freeze assets, Washington will also squeeze Zarif's ability to function as a globe-trotting diplomat. He is expected to be able to continue to visit the United Nations in New York, albeit under tight restrictions.

"The United States will continue to uphold our obligations," the administration official said.

In mid-July, Secretary of State Mike Pompeo said that Zarif would be issued a visa but would be limited to the area reaching six blocks around the UN headquarters.

Trump last year left the 2015 denuclearisation accord negotiated by Zarif with six nations including the United States under former president Barack Obama and imposed crippling sanctions on Tehran's economy, also vowing to curb Tehran's regional influence.

With tensions escalating in June, Trump says he issued a last-minute cancellation of military strikes on Iran after it shot down a US drone.

Critics question the legal or diplomatic rationale for targeting Zarif, however, saying that sanctions will all but end the possibility of dialogue.

"If you sanction diplomats you'll have less diplomacy," Republican Senator Rand Paul tweeted.

The senior administration official insisted that Washington still wants talks — only not with Zarif.

US diplomats "do not consider him to be our primary point of contact," the official said.

"If we do have an official contact with the Iranians, we want someone who is a significant decision maker."

"The United States continues to seek a diplomatic solution that addresses the Iranian regime's destructive behaviour. The only path forward is a comprehensive deal that addresses the full range of its threats," Pompeo said in a statement.

"Until then, our campaign of diplomatic isolation and maximum economic pressure will continue," he said.

Zarif, meanwhile, sought to minimise the sanctions' impact on him, tweeting that they have "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."