The U S will unleash its “toughest ever” sanctions against Iran on Monday following a wave of protests across the oil-rich country.
The Trump administration is reinstating all sanctions removed under the 2015 nuclear deal, targeting both Iran and states that trade with it.
It will hit oil exports, shipping and banks, all core parts of the economy.
Thousands of Iranians chanting “Death to America” rallied on Sunday, rejecting calls for talks.
Iran’s military was also quoted as saying it would hold air drills on Monday and Tuesday to prove the country’s defensive capabilities.
The demonstrations took place on the 39th anniversary of the occupation of the U S embassy in Tehran, which led to four decades of mutual hostility.
Before travelling to a campaign rally for the U S mid-term elections, President Donald Trump said Iran was already struggling under his administration’s policies.
“The Iran sanctions are very strong. They are the strongest sanctions we’ve ever imposed. And we’ll see what happens with Iran, but they’re not doing very well, I can tell you.”
Washington is re-imposing the sanctions after Mr Trump in May pulled out of a 2015 accord aimed at curbing Iran’s nuclear ambitions.
Washington also says it wants to stop what it calls Tehran’s “malign” activities including cyber attacks, ballistic missile tests, and support for terror groups and militias in the Middle East.
“We are working diligently to make sure we support the Iranian people and that we direct our activity towards ensuring that the Islamic Republic of Iran’s malign behaviour is changed,” U S Secretary of State, Mike Pompeo, told Fox News Sunday.
“That’s the goal, that’s the mission, and that’s what we will achieve on behalf of the president.”
The U S has been gradually re-imposing sanctions, but analysts say this latest round is by far the most significant.
More than 700 individuals, entities, vessels and aircraft will be put on the sanctions list, including major banks, oil exporters and shipping companies.
Mr. Pompeo has said that over 100 big international companies had withdrawn from Iran because of the looming sanctions.
He also said Iranian oil exports had dropped by nearly one million barrels a day, choking the main source of funding for the country.
In addition, the Brussels-based Swift network for making international payments is expected to cut off links with targeted Iranian institutions, isolating Iran from the international financial system.
The UK, Germany and France – which are among the five countries still committed to the nuclear pact – have all objected to the sanctions.
They have promised to support European firms that do “legitimate business” with Iran and have set up an alternative payment mechanism – or Special Purpose Vehicle (SPV) – that will help companies trade without facing U S penalties.
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Mexico City receives first wave from migrant caravan
The first migrants from a large convoy of people travelling through Mexico towards the United States have arrived in the capital, Mexico City.
About 450 people, mostly men and boys, have been given temporary shelter at a sports stadium.
The convoy, known as a caravan, set off from Honduras several weeks ago and is now about 5,000 strong.
US President Donald Trump says troops will be deployed to stop them crossing the US-Mexican border.
Critics have accused Mr. Trump of using the threat of illegal immigration to fire up his supporters ahead of Tuesday’s mid-term elections.
The migrants says they are fleeing persecution, poverty and violence in their home countries of Honduras, Guatemala, and El Salvador.
Many of those arriving in the Jesus Martinez Palillo stadium on Sunday had walked or hitched rides from the state of Veracruz.
“From what I brought from Honduras I have nothing. Even yesterday I walked barefoot,” Kenia Alvarado, 21, told El Universal newspaper after arriving at the stadium.
Veracruz Governor Miguel Angel Yunes said on Friday that buses would be provided to take migrants to the capital – but the offer was later withdrawn.
“Our heads are set at getting to the United States, to fulfil the American dream,” said Mauricio Mancilla, who had travelled from northern Honduras with his six-year-old son. “We have faith in God that we will do this, whatever the circumstances.”
Authorities in Mexico City say they have prepared food and shelter as well as medical services and legal advisers for the thousands who are expected to arrive in the coming weeks.
Tanks capable of holding 10,000 litres (2,200 gallons) of drinking water have been installed in the stadium, La Jornada reported.