Iran's oil exports could fall by as much as two-thirds by the end of the year because of the USA sanctions, putting oil markets under huge strain amid supply outages elsewhere in the world.
Trump's policies are already putting significant pressure on the Iranian economy, although US intelligence suggests they may ultimately rally Iranians against the United States and strengthen Iran's hardline rulers, officials say.
Sporadic protests broke out in several cities in Iran for a fifth night on Saturday, a day after demonstrators attacked a seminary, according to Iranian news agencies and social media, as Iranians brace for a return of U.S. sanctions.
But all Iranians are concerned about the struggling economy, especially since the United States walked out of the nuclear deal in May and announced it would reimpose full sanctions in two stages. Iran's powerful Revolutionary Guard on Sunday acknowledged conducting recent naval exercises near the crucial Strait of Hormuz after renewing threats to cut off the waterway to oil traders.
Iran is also said to have started to offer India cargo insurance and tankers operated by Iranian companies as some Indian insurers have refused to cover oil cargoes from Iran in the face of the returning USA sanctions on Tehran.
"The Americans say they want to reduce Iranian oil exports to zero".
The rhetoric between the USA and Iran remains heated, despite Trump tweeting last week that he would be willing to meet with Iranian President Hassan Rouhani. "It shows they have not thought about its consequences", Rouhani was quoted as saying by state news agency IRNA while on an official visit to Austria's capital, Vienna, in July.
The first phase hits on Tuesday with blocks on financial transactions and imports of raw materials, as well as sanctions on Iran's automotive sector and commercial aircraft purchases.
The Trump administration had wanted Iranian oil exports cut to zero by November 4.
On 7 August, Washington will reimpose sanctions on Iran's purchase of United States dollars, its trade in gold and precious metals and its dealings with metals, coal and industrial-related software.