The US government on Wednesday imposed financial sanctions on Iran's foreign minister, Mohammad Javad Zarif, as part of its escalating campaign of pressure against the Islamic Republic.

The highly unusual action of penalizing the top diplomat of another nation comes a month after President Donald Trump signed an executive order placing sanctions on Iran's supreme leader, Ayatollah Ali Khamenei.

"Javad Zarif implements the reckless agenda of Iran’s Supreme Leader, and is the regime’s primary spokesperson around the world," Treasury Secretary Steven Mnuchin said in a statement announcing the sanctions.

Zarif responded swiftly to the US move, stating in a tweet that the sanctions would have “no effect on me or my family, as I have no property or interests outside of Iran”.

He added: “Thank you for considering me such a huge threat to your agenda.”

US officials say Zarif's travels to New York for official UN business would be uninhibited by the new sanctions, in accordance with America's international obligations.

Zarif met earlier this month with Republican Senator Rand Paul, a Trump ally, about potentially opening a back channel for restarting nuclear talks between the two countries.