The US special envoy for Iran says Washington will sanction any country that imports Iranian oil and there are no exemptions in place.

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," 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.

"There are right now no oil waivers in place," Hook told reporters in London.

"We will sanction any illicit purchases of Iranian crude oil."

Tehran has been selling increased volumes of petrochemical products at below-market rates in countries including Brazil, China and India since the United States reimposed sanctions on Iranian oil exports in November, Reuters reported this month.

"Iran does have a history of using front companies to evade sanctions and enrich the regime and fund its foreign adventurism," Hook said, adding that Iran routinely violates maritime law to hide its oil exports.

Last year, Trump pulled the US out of the multinational deal under which sanctions on Iran were lifted in return for curbs on its nuclear programme, verified by the International Atomic Energy Agency (IAEA).

Washington has since re-imposed tough sanctions on Iran, aiming to cut the Islamic Republic's oil sales to zero to force it to negotiate a broader deal that would also cover its ballistic missile capabilities and regional influence.

Iran wants to sell its oil at the same levels that it did before Washington withdrew from the accord.

"Iran has just rejected diplomacy too many times," Hook said.

"They have got to stop this sectarian agenda of creating a Shia corridor of power to dominate the Middle East."

"This is a clerical regime that wants to remake the Middle East in its image - and that would deeply destabilise the Middle East to have regimes following the same Marxist theocratic regime," he added.