Washington said Wednesday that it was imposing sanctions on seven firms it accused of selling Iranian oil -- four based in the United Arab Emirates and one in Turkey.

"So long as Iran attempts to generate oil and petrochemical revenues to fund its destabilising activities and support its terrorist activities and proxies, the United States will take steps to hold both Iran and all its partners engaged in sanctions evasion accountable," Secretary of State Marco Rubio said.

Iran's foreign ministry said the new sanctions were the latest US attempt "to disrupt friendly and legal relations between developing countries through economic terrorism".

Ministry spokesman Esmaeil Baqaei said they provided "clear evidence of the contradictory approach of American decision-makers and their lack of goodwill and seriousness in advancing the path of diplomacy".

Iran and the United States have held three rounds of talks brokered by Oman since April 12 seeking a new nuclear agreement to replace one abandoned by President Donald Trump during his first term in 2018.

A fourth round had been due to be held in Rome on Saturday but Omani foreign minister Badr Albusaidi announced on Thursday that it had been postponed to a later date.

"For logistical reasons we are rescheduling the US-Iran meeting provisionally planned for Saturday May 3. New dates will be announced when mutually agreed," Albusaidi said in a statement on X.

Baqaei confirmed the fourth round had been postponed, saying the decision had been taken "based on the proposal of the Omani minister".

Since returning to office in January, Trump has reinstated his first term policy of "maximum pressure" against Iran.

He has coupled the opening of negotiations with Iran for a new nuclear deal with threats of military action if they fail.