The first set of US sanctions against Iran that had been eased under the landmark nuclear accord have gone back into effect under an executive order signed by President Donald Trump.
The sanctions target financial transactions that involve US dollars, Iran's automotive sector, the purchase of commercial planes and metals including gold.
More US sanctions targeting Iran's oil sector and central bank are to be reimposed in early November.
In an early-morning tweet, Trump said the re-imposition of sanctions mean, "Anyone doing business with Iran will NOT be doing business with the United States."
"I am asking for WORLD PEACE, nothing less!"
Trump pulled the US out of the international accord limiting Iran's nuclear activities three months ago, declaring that the landmark 2015 agreement was "horrible" and left the Iranian government flush with cash to fuel conflict in the Middle East.
But European Union foreign policy chief Federica Mogherini, during a visit to Wellington, New Zealand, said that the EU is encouraging enterprises to increase their business with Iran, as the country has been compliant with their nuclear-related commitments
"We are doing our best to keep Iran in the deal, to keep Iran benefiting from the economic benefits that the agreement brings to the people of Iran because we believe this is in the security interests of not only our region, but also of the world," she said. "If there is one piece of international agreements on nuclear non-proliferation that is delivering, it has to be maintained."
A senior administration official, briefing reporters under ground rules requiring anonymity, said the United States is "not particularly concerned" by EU efforts to protect European firms from the sanctions.
Iranian President Hassan Rouhani said that Iran still can rely on China and Russia to keep its oil and banking sectors afloat. Speaking in a television interview, he also demanded compensation for decades of American "intervention" in the Islamic Republic.
Months of uncertainty surrounding the sanctions have already further hurt Iran's economy. The country's rial currency has tanked, and the downturn has sparked protests across the nation.
The "Trump Administration wants the world to believe it's concerned about the Iranian people," Iranian Foreign Minister Mohammad Javad Zarif said in a statement posted to Twitter. But, he said, the reimposed sanctions would endanger "ordinary Iranians".
"US hypocrisy knows no bounds," he said.
Israeli Prime Minister Benjamin Netanyahu, a firm foe of the Iranian government, said the sanctions symbolise "the determination to block Iran's regional aggression as well as its continuous plans to arm itself with nuclear weapons".