On the supply-side, oil is in ample availability despite the sanctions against Iran as output from the world's top-three producers, Russian Federation the United States and Saudi Arabia, is rising. He said Iran was continuing its pursuit of nuclear weapons, and that the sanctions relief gave Iran cash to carry out destabilizing activities in the Middle East.
"Treasury's imposition of unprecedented financial pressure on Iran should make clear to the Iranian regime that they will face mounting financial isolation and economic stagnation until they fundamentally change their destabilising behaviour", Mr Mnuchin said in a statement.
India, which is the second biggest buyer of Iranian oil after China, is being pushed by the U.S. to restrict its monthly purchase to 1.25 million tonnes or 15 million tonnes in a year (300,000 barrels per day), down from 22.6 million tonnes (452,000 barrels per day) bought in 2017-18 financial year, according to sources. However, the US has granted eight temporary waivers to countries that will let them wind down their imports over time.
The deal saw Iran agree to limits on its nuclear programme in exchange for the lifting of sanctions.
In August, the European Commission revamped its Blocking Statute, a 1995 law created to help European companies and banks recover damages arising from USA sanctions on third parties. At the same time, the United States temporarily exempted eight nations, including Japan, from the sanctions on importing oil from Iran. China has publicly defied the U.S. call for curbs on oil imports from Iran.
The Belgium-based SWIFT financial messaging service said it is suspending some unspecified Iranian banks' access to its messaging system in the interests of the stability and integrity of the global financial system.
But they were opposed by other signatories to the 2015 nuclear deal - Britain, France, Russia, China, Germany and the EU.
The US stance has already inflicted serious pain on Iranians, with the country's currency, the rial, losing more than two thirds of its value since May.
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"What the Americans are doing today is putting pressure merely on the people", he said, according to a transcript of the remarks posted on the president's website.
Oil markets have been anticipating the sanctions for months and the world's biggest producers have been increasing output.
It then announced that eight nations, including Korea, had received waivers and could continue imports of Iranian crude for 180 days on the condition that they significantly reduce the volume of imports from Iran during that period.
U.S. West Texas Intermediate (WTI) crude futures were down 35 cents, or 0.6 percent, at $62.79 a barrel.
News of waivers on the sanctions weighed on prices, analysts said. The fear was that the sanctions deadline would have pushed oil prices sharply higher. "We are standing up to a bullying enemy", said Iranian President Hassan Rouhani. Many should choose the United States, or have already done so.
Oil is Iran's main source of income.
President Donald Trump has called the agreement a "disaster" and, as of Monday, the United States will sanction countries and companies that do business with Iran or buy its oil.
Since May, Europeans have said repeatedly that they want to preserve the nuclear deal and have focused their diplomatic efforts on keeping trade alive with Iran however possible.