President Donald Trump said on Saturday that the United States would impose “major” new sanctions on Iran in two days, just hours after saying he would be the Islamic republic's new “best friend” if they were to renounce nuclear arms.

Tensions between the United States and Iran have soared this week after Tehran downed a US surveillance drone.

Trump said on Friday that he called off retaliatory military strikes at the last minute because the response was not “proportionate.” But he had brandished the threat of sanctions ever since, and has now set a timetable.

“We are putting major additional Sanctions on Iran on Monday,” Trump tweeted, while adding: “I look forward to the day that Sanctions come off Iran, and they become a productive and prosperous nation again - The sooner the better!”