LAUSANNE, Switzerland — The United States and Iran plunged back into negotiations Sunday, hoping to end a decades-long standoff that has raised the specter of an Iranian nuclear arsenal, a new atomic arms race in the Middle East and even a U.S. or Israeli military intervention.

Two weeks out from a deadline for a framework accord, some officials said negotiators would likely settle for an announcement that they’ve made enough progress to justify further talks.

Such a declaration would hardly satisfy American critics of the Obama administration’s diplomatic outreach to Iran and hardliners in the Islamic Republic, whose rumblings have grown more vociferous and threatening as the parties have narrowed many of their differences.

And, officially, the United States and its partners insist their eyes are on a much bigger prize: “A deal that would protect the world,” Secretary of State John Kerry said this weekend, “from the threat that a nuclear-armed Iran could pose.”

Yet as Kerry arrived in Switzerland for several days of discussions with Iranian Foreign Minister Mohammad Javad Zarif, no one was promising the breakthrough.

One diplomat said new differences surfaced only in the last negotiating round of what has been a 15-month process, including a sudden Iranian demand that a nuclear facility buried deep underground be allowed to keep hundreds of centrifuges that are used for enriching uranium – material that can be used in a nuclear warhead.

Previously, the Iranians had accepted that the plant would be transformed into one solely for scientific research, that diplomat and others have said.