U.S. President Donald Trump said all purchases of Iranian oil or petrochemical products must stop and any country or person buying any from the country would be immediately subject to secondary sanctions.
"They will not be allowed to do business with the United States of America in any way, shape, or form," he wrote on Truth Social on Thursday.
Trump's comments follow the postponement of the latest U.S. talks with Iran over its nuclear program, which had been due to take place in Rome on Saturday. A senior Iranian official told Reuters a new date will be set "depending on the U.S. approach."
Trump's administration has targeted Tehran with a series of sanctions on entities including a China-based crude oil storage terminal and an independent refiner it has accused of being involved in illicit trade in oil and petrochemicals.
In February Trump restored a "maximum pressure" campaign on Iran which includes efforts to drive its oil exports to zero and help prevent Tehran from developing a nuclear weapon.
Secondary sanctions are those where one country seeks to punish a second country for trading with a third by barring access to its own market, a particularly powerful tool for the United States because of the size of its economy.
Analysts have said that to really crack down on Iran's oil exports the U.S. would have to impose secondary sanctions on entities such as Chinese banks that facilitate the purchases of Iranian oil. China is the largest buyer of Iranian crude.