Mr. Trump’s decision unravels the signature foreign policy achievement of his predecessor, Barack Obama, isolating the United States among its allies and leaving it at even greater odds with its adversaries in dealing with the Iranians.

The United States is preparing to reinstate all sanctions it had waived as part of the nuclear accord — and impose additional economic penalties as well, the person said.

A second person familiar with negotiations to keep the 2015 accord in place said the talks collapsed over Mr. Trump’s insistence that sharp limits be kept on Iran’s nuclear fuel production after 2030. The deal currently lifts those limits.