The United States will sanction any country that imports Iranian oil and there are no exemptions in place, the US special envoy for Iran said on Friday.

US President Donald Trump targeted Iranian Supreme Leader Ayatollah Ali Khamenei and other top Iranian officials with sanctions on Monday, taking an unprecedented step to increase pressure on Iran after Tehran's downing of an unmanned American drone last week.

“We will sanction any imports of Iranian crude oil... There are right now no oil waivers in place,” Brian Hook said when asked about the sale of Iranian crude to Asia, adding that the US would take a look at reports of Iranian crude going to China.

“We will sanction any illicit purchases of Iranian crude oil,” Hook told reporters in London.