Asked how he would respond if Tehran restarted its nuclear efforts, Trump threatened that "Iran will find out."

"I would advise Iran not to start their nuclear program; I would advise them very strongly," the US president told reporters at the White House. "If they do, there will be very severe consequences."

Trump's move undercut more than a decade and a half of diplomacy by Britain, China, France, Germany, Iran, Russia and past US administration, which had resulted in the deal that lifted sanctions on Iran in exchange for limits on its nuclear program.

Iranian President Hassan Rouhani had warned his country could resume uranium enrichment "without limit" but would refrain from doing so for now.

Other world powers have vowed to uphold the landmark deal, which places restrictions on Iran's nuclear program and subjects it to international inspections.

US withdrawal, however, puts at risk billions of dollars of foreign investment in Iran and is likely to sharpen tensions in the Middle East.

In announcing the US pullout Tuesday, Trump dismissed the accord as an "embarrassment" that was "defective at its core" and called for a "new and lasting deal."