US President Donald Trump has issued a strong warning to anyone trading with Iran, following his re-imposition of sanctions on the country.
"Anyone doing business with Iran will NOT be doing business with the United States," the president tweeted.
Some re-imposed sanctions took effect overnight and tougher ones relating to oil exports will begin in November.
Iran's president said the measures were "psychological warfare" which aimed to "sow division among Iranians".
The sanctions follow the US withdrawal from the Joint Comprehensive Plan of Action, also known as the Iran nuclear deal, earlier this year.
The Iran sanctions have officially been cast. These are the most biting sanctions ever imposed, and in November they ratchet up to yet another level. Anyone doing business with Iran will NOT be doing business with the United States. I am asking for WORLD PEACE, nothing less!— Donald J. Trump (@realDonaldTrump) August 7, 2018
The deal, negotiated during the presidency of Barack Obama, saw Iran limit its controversial nuclear activities in exchange for sanctions relief.
Mr Trump has called the deal "one-sided", "disastrous" and the "worst I've ever seen". He believes renewed economic pressure will force Iran to agree to a new deal.
The European Union, which remains committed to the original agreement, has spoken out against the sanctions, vowing to protect firms doing "legitimate business" with Iran.
He praised the "most biting sanctions ever imposed" and said they would "ratchet up to another level" in November.
"I am asking for WORLD PEACE, nothing less!" he said.
Earlier this week, he had said that Iran faced a choice to "either change its threatening, destabilising behaviour and reintegrate with the global economy, or continue down a path of economic isolation".
"I remain open to reaching a more comprehensive deal that addresses the full range of the regime's malign activities, including its ballistic missile programme and its support for terrorism," he said.
Iranian President Hassan Rouhani said the US government had "turned their back on diplomacy".
"They want to launch psychological warfare against the Iranian nation." he said. "Negotiations with sanctions doesn't make sense. We are always in favour of diplomacy and talks... but talks need honesty."
The foreign ministers of Germany, the UK and France released a statement on Monday that said the nuclear deal remained "crucial" to global security.
They also unveiled a "blocking statute", which is intended to protect European firms doing business with Iran despite the new US sanctions.
Alistair Burt, the UK's minister of state for the Middle East, told the BBC: "If a company fears legal action taken against it and enforcement action taken against it by an entity in response to American sanctions, then that company can be protected as far as EU legislation is concerned."
He said Iran would simply "batten down the hatches" until the next US election.
However, German car and lorry maker Daimler, which announced a joint venture in Iran last year, confirmed this week that it has now ceased activities in the country.
What are the sanctions?
Mr Trump signed an executive order that brought sanctions back into place yesterday. They target:
- The purchase or acquisition of US banknotes by Iran's government
- Iran's trade in gold and other precious metals
- Graphite, aluminium, steel, coal and software used in industrial processes
- Transactions related to the Iranian rial currency
- Activities relating to Iran's issuance of sovereign debt
- Iran's automotive sector
- A second phase is planned to come back into effect on 5 November which will have implications for Iran's energy and shipping sectors, petroleum trading and transactions by foreign financial institutions with the Central Bank of Iran.