US President Donald Trump on Thursday pledged to punish countries that buy oil from Iran, implicitly threatening new sanctions on China.

“Any country or person who buys any amount of oil or petrochemicals from Iran will be subject to, immediately, secondary sanctions,” Mr Trump wrote on social media.

He said any countries who continue to buy Iranian oil would be prohibited from doing business with the US “in any way, shape or form”.

Although Mr Trump did not mention China by name in his post, the State Department said Beijing “is by far the largest importer of Iranian oil”.

Washington has accused Tehran of using revenue it receives from oil to finance attacks on US allies, support terrorism and pursue other destabilising actions.

China is subject to a 145 per cent US tariff, with Mr Trump accusing Beijing of taking advantage of American trade and trying to push the Chinese government into cracking down on fentanyl production.

While a member of Opec, Iran was not subject to the bloc’s voluntary production cuts because of US sanctions on Tehran.

Opec+ announced in April that it will increase output by 411,000 barrels per day this month, pointing to “continuing healthy market fundamentals and the positive market outlook”.

The latest threat by the President comes during his so-called maximum pressure campaign on Iran, in which Washington is aiming to bring down its oil exports to “zero”.

Mr Trump’s administration is in discussions with Tehran over a nuclear deal. A fourth round of talks between the two that had been scheduled for Saturday has been postponed, mediator Oman said earlier on Thursday.

Mr Trump in March also threatened he would impose a 25 per cent tariff on any country that buys oil from Venezuela, another Opec member, over unsubstantiated allegations that the country deliberately sent criminals and gang members into the US.