"I am announcing the harshest economic action ever taken against any country. This will be economic warfare and isolation on a scale never seen before," the president wrote on social media.
He also promised "unprecedented" economic penalties for countries that help Tehran.
"This will be an economic D-Day," he wrote, urging all allies to side with the United States to "isolate and defeat" the threat from Iran.
The move comes a few days after a 60-day ceasefire agreed between Iran and the US in June expired. The idea was that a longer-term peace solution to the Iran war would be negotiated during it, but that has not happened.
