"They're the strongest sanctions we've ever imposed", said President Donald Trump en route to a rally in Georgia on Sunday evening, hours before the sanctions on Iran's energy, shipbuilding and banking sectors kicked in at midnight.
Swift now needs to decide whether or not to abide by the USA demand - a decision that could involve breaking European Union rules - although it may well be that banks will now shun Iranian transactions whatever Swift decides.
But the sanctions on Iranian oil exports also reportedly exempt several of the world's largest economies. "Rest assured, Iran will never get close to obtaining a nuclear weapon under President Trump's watch", he said. "We are in the economic war situation".
"We are at an economic war situation".
"US bullying is backfiring".
"We've said for a long time, zero should mean zero".
The United States says the sanctions are not aimed at toppling the government, but at persuading it to radically change its policies, including its support for regional militant groups and its development of long-range ballistic missiles. Later that day, National Security Adviser John Bolton announced the administration was launching a review of all global treaties that could lead to more rulings by the U.N.'s principal judicial organ.More news: India's war on red tape is working, says World Bank
Details of the sanctions will be released at a news conference scheduled for 8:30 a.m. EST with Pompeo and U.S. Treasury Secretary Steven Mnuchin.
China is a signatory to the 2015 Joint Comprehensive Plan of Action - which includes Britain, France, Germany and Russian Federation - that Trump pulled out from earlier this year.
Late last week, USA government officials signaled that the United States had granted waivers to eight countries to continue temporary buying Iranian oil, on the condition that they had significantly reduced purchases from Iran.
Washington granted waivers to eight countries.
Iranian leaders called the sanctions "illegal" and said they would only hurt ordinary people. But Brent crude was steady at $72.83 by 1230 GMT.
West Texas Intermediate crude for December delivery dropped as much as 1 per cent to US$62.52 a barrel on the New York Mercantile Exchange, the lowest since April 9, and traded at US$62.87 as of 10:35 a.m. London time.
"I am very confident that the sanctions that will be reimposed this Monday - not only the crude oil sanctions but the financial sanctions are being put in place by the Treasury Department and over 600 designations of individuals and companies in Iran - will have the intended effect to alter the Iranian regime's behavior", Pompeo said "That's our expectation". In granting waivers, Washington is trying to ensure that global oil prices don't spike, especially in the middle of the USA election season.
In a speech on Saturday, Iran's Supreme Leader Ayatollah Khamenei said Mr Trump's policies faced opposition around the world.
While the sheer number of people and entities sanctioned is larger than ever, many Middle East experts believe they will be less effective than the United Nations sanctions in place before the deal.