"The short-term authorization permitting the sale of Iranian oil already stranded at sea is set to expire in a few days and will not be renewed," the Treasury Department said in a statement.

The initial authorization allowed for the delivery and sale of Iranian crude and other petroleum products loaded onto ships before March 20, and was to last through April 19.

The Treasury Department added that it is "maintaining maximum pressure" on Tehran.

A senior US official, speaking on condition of anonymity, told AFP that the Trump administration is sending letters to China, Hong Kong, the United Arab Emirates, and Oman, pointing to banks in their jurisdictions believed to have permitted Iranian illicit activity.

The letter, seen by AFP, said the Treasury Department is seeking authorities' cooperation to halt any illicit activity linked to Iran.

It adds that it has reason to believe banks in these jurisdictions processed illicit Iranian funds.

The temporary easing on Iranian oil previously was part of a series of measures launched by the Trump administration to quell skyrocketing energy prices following the start of the war on February 28, including a similar easing of sanctions on Russian oil at sea.

Iran, in response to the US-Israeli attacks, effectively blocked the flow of vessels through the critical Strait of Hormuz, through which one-fifth of global oil and gas supplies flow.

While the United States and Iran have agreed to a two-week ceasefire, recent talks in Pakistan failed to achieve a breakthrough, and President Donald Trump subsequently ordered a blockade of Iranian ports to increase pressure on Tehran.