The unilateral sanctions reintroduce some of the most crippling restrictions on Iran’s oil, shipping and banking sectors and seek to penalise even non-US entities that do business with Iran.

Iranian leaders called the sanctions “illegal” and said they would only hurt ordinary people.

Iran’s economy has faced stagnant growth and high unemployment, even after sanctions were lifted following the nuclear deal it negotiated with world powers.

In recent months, its currency has plummeted, raising prices and wiping out savings.

“We have to make Americans understand that they cannot talk to the great Iranian nation with the language of pressure and sanctions,” Rowhani said Monday in televised remarks.

He spoke to a conference of economists, who he said were at the “forefront of the resistance” against the United States.

“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.

Also Monday, Iran’s military and its powerful Revolutionary Guard Corps staged joint war drills in the northern and western parts of the country, the official Islamic Republic News Agency said. The exercises include air defence systems and anti-aircraft batteries.

US Secretary of State Mike Pompeo and other administration officials have described the penalties as the “toughest sanctions ever placed” on Iran.

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 UN sanctions in place before the deal.

That is because virtually every country in the world was behind the previous sanctions, while all but a handful of nations oppose their reimposition.

The most significant of the new measures is a prohibition against oil and gas sales, which provide the Iranian government with 80 per cent of its total revenue.

The blacklisted companies include 50 Iranian banks, an Iranian airline and dozens of its planes, as well as officials and vessels in Iran’s shipping and energy sectors.

US President Donald Trump withdrew from the 2015 nuclear deal in May and gave nations and businesses 180 days to wind down their oil purchases to “zero”.

The administration has granted waivers to eight countries that have reduced their oil purchases from Iran but not stopped them entirely.

The countries allowed to keep buying oil from Iran temporarily under the sanctions include China’s two biggest oil customers, China and India, Pompeo announced Monday. Also granted waivers were Italy, Greece, Japan, South Korea, Turkey and Taiwan.

China imported an average of 718,000 barrels a day from Iran in the first five months of this year, accounting for about a third of Iran’s oil exports, according to Chinese government data.

In addition, Pompeo said the United States has granted waivers to continue three non-proliferation projects that provide oversight on Iran’s nuclear programme.

The only one he identified was in Bushehr, where Russia is building a second unit at an existing nuclear power plant.

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.

In August, the European Commission revamped its Blocking Statute, a 1995 law designed to help European companies and banks recover damages arising from US sanctions on third parties.

The legislation also implies that European courts could nullify US decisions regarding sanctions.

However, many European companies with a US presence remain cautious about those European tools, whose effectiveness has yet to be tested.

Reacting on Twitter, Iranian Foreign Minister Mohammad Javad Zarif said that “US bullying is backfiring.”

He added: “The US – & not Iran – is isolated.”

In withdrawing from the nuclear deal, the Trump administration complained that it did not go far enough in restricting Iran’s nuclear programme and did not cover other activities it finds objectionable.

Under the deal, Iran curbed its atomic energy programme in exchange for broad relief from nuclear-related sanctions. Iran has complied with the terms of the nuclear deal, according to the International Atomic Energy Agency, the United Nations’ nuclear watchdog tasked with monitoring the country’s nuclear activity.

But the Trump administration demands that Iran change its “malign behaviour” in the region, including ballistic missile development and support for regional proxies.

The US pull-out has handed a victory to Iranian hardliners, who opposed the nuclear deal on grounds it gave away too much to the United States and five other world powers that signed it. The staunchly anti-American hardliners have been the main drivers of Iran’s support for militant groups in Syria, Lebanon and Iraq.

Iran experts warn, however, that sanctions are unlikely to alter Iranian influence or activities in the region.

A report released Friday by the Brussels-based International Crisis Group tracked Iran’s economic performance and regional policy over four decades and concluded that there was “little to no correlation between the two”.

“Tehran has continued to pursue policies it deems central to its national security no matter its degree of economic well-being at home,” the report said.