Iran's President Hassan Rouhani said Thursday a US decision to impose sanctions on Iranian Foreign Minister Mohammad Javad Zarif shows they are "afraid" of his top diplomat, as tensions heightened between the arch-enemies.

The US Treasury said the sanctions would freeze any of Zaria's assets in the US or controlled by US entities, as well as squeeze his ability to function as a globe-trotting diplomat.

"They are afraid of our foreign minister's interviews," Rouhani said in a televised speech, referring to a recent round of interviews Zarif gave to foreign 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. Maybe there's no better way to describe (the sanctions) but childish," Rouhani said on a visit to the northwestern city of Tabriz.

The designation of Zarif under the same sanctions earlier applied to Iran's supreme leader Ayatollah Ali Khamenei is the latest in a string of moves by the US against the Islamic republic.

"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.

"The United States is sending a clear message to the Iranian regime that its recent behavior is completely unacceptable."

The arch-foes have been locked in a battle of nerves since President Donald Trump withdrew the US from a landmark 2015 deal aimed at limiting Iran's nuclear program and began reimposing sanctions. The situation has worsened since the Trump administration stepped up a campaign of "maximum pressure" against Iran this year, with drones downed and tankers mysteriously attacked in Gulf waters.

China said Wednesday it opposes unilateral sanctions and the so-called "long-arm jurisdiction" of the US, and urged the US to avoid taking actions that further fuel tensions. 

At Thursday's press briefing, Chinese Foreign Ministry spokesperson Hua Chunying said that "the unilateral sanctions and so-called 'long-arm jurisdiction'" will not help solve the Iranian nuclear issue, and dialogue and negotiations are the fundamental ways to solve the issue.

"The US has repeatedly expressed its willingness to engage in unconditional dialogue with Iran. We hope the US can match its words with deeds and do things conducive to regional peace and stability," Hua said.