The US has intensified its stance against Iranian oil trade, with President Donald Trump announcing that any purchases of Iranian oil or petrochemical products must cease.

The President warned that secondary sanctions would be imposed on any country or entity engaging in such transactions, effectively barring them from doing business with the US, according to a report by Reuters.

"They will not be allowed to do business with the United States of America in any way, shape, or form," Trump was quoted as saying on Truth Social.

This move follows the postponement of nuclear programme talks with Iran, originally scheduled to be held on Saturday, which have been delayed with no new date confirmed, according to a senior Iranian official.

The Trump administration has reinstated a "maximum pressure" campaign aimed at reducing Iran's oil exports to zero and preventing the development of a nuclear weapon, the report said.

This includes sanctions on a China-based crude oil storage terminal and an independent refiner accused of engaging in illicit oil and petrochemical trade.

Secondary sanctions, a potent economic tool, allow the US to penalise countries that trade with targeted nations by denying them access to the US market.

In October last year, the US Government imposed sanctions on Iran's petroleum and petrochemical sectors after a missile strike on Israel. The sanctions were intended to limit Iran's funding for regional destabilisation activities.

The Department of State had sanctioned six entities and identified six vessels as blocked property for their involvement in the Iranian petroleum trade.

Amidst these developments, Russia and Iran have reportedly reached a preliminary agreement for annual gas supplies of 55 billion cubic metres, with pricing discussions ongoing.

Russia has also agreed to finance the construction of a new nuclear power plant in Iran.