Donald Trump has warned countries doing business with Iran that they will not be doing business with the US.
Mr Trump also said in an early morning Twitter session that the sanctions placed back on Iran by the US were reintroduced in his pursuit for world peace.
The first set of US sanctions against Iran that had been eased under the landmark nuclear accord went back into effect early on Tuesday under an executive order signed by Mr Trump.
They targeted 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.
Mr Trump tweeted: "Anyone doing business with Iran will NOT be doing business with the United States."
He added: "I am asking for WORLD PEACE, nothing less!"
The stiff economic sanctions increase pressure on the Islamic Republic despite statements of deep dismay from European allies, three months after Mr Trump pulled the US out of the international accord limiting Iran's nuclear activities.
Mr Trump declared the landmark 2015 agreement had been "horrible", leaving the Iranian government flush with cash to fuel conflict in the Middle East.
Iran accused the US of reneging on the nuclear agreement, signed by the Obama administration, and of causing recent Iranian economic unrest. European allies said they "deeply regret" the US action.
As the sanctions loomed, Mr Trump said in a statement: "We urge all nations to take such steps to make clear that the Iranian regime faces a choice: either change its threatening, destabilising behaviour and reintegrate with the global economy, or continue down a path of economic isolation."
Mr Trump warned that those who don't wind down their economic ties to Iran "risk severe consequences".