LAUSANNE,  Switzerland (AP) – The United States and Iran are plunging back into negotiations in a bid 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 persistent differences meant 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 emphasized this past 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 the plant would be transformed into one solely for scientific research, that diplomat and others have said.

The deal that had been taking shape would see Iran freeze its nuclear program for at least a decade, with restrictions then gradually lifted over a period of perhaps the following five years. Washington and other world powers would similarly scale back sanctions that have crippled the Iranian economy in several phases. Iran says it is only interested in peaceful energy generation and medical research, but much of the world has suspected it of maintaining covert nuclear weapons ambitions. And the U.S. and its ally Israel have at various times threatened military action if Iran's program advances too far.

Speaking Sunday on CBS News, Kerry said most of the differences between Iran and the negotiating group of the U.S., Britain, China, France, Germany and Russia were ``political,'' not technical. He didn't elaborate, but political matters tend to include levels of inspections, Iran's past military work linked to its nuclear program and how quickly to scale back sanctions. Technical matters refer, for example, to how many centrifuges Iran can maintain, what types of those machines and how much plutonium it would be allowed to produce from a planned heavy water reactor.

Less than four months ago, senior officials talked optimistically about reaching a preliminary agreement by March, with three months of additional talks only for any remaining technical work. Back then, British Foreign Secretary Philip Hammond said he expected ``an agreement on substance'' by March 31. Top Western and Iranian negotiators issued a joint statement vowing to use the time until June 30 only ``if necessary ... to finalize any possible remaining technical and drafting work.''

But two diplomats said ahead of this week's talks in the Swiss city of Lausanne that persistent differences at the negotiating table had diminished the chances of such a substantial agreement. Instead, they said, the sides were more likely to restrict themselves to a vague oral statement indicating that enough headway had been made to continue negotiations. They weren't authorized to speak publicly about the sensitive talks and demanded anonymity.

A senior U.S. official rejected that assessment, saying negotiators were aiming for substantive achievements. Top diplomats and technical experts from the U.S. and Iran met Sunday. Kerry and Zarif held their first discussion Monday morning.

Anything short of a written agreement will only encourage congressional critics of the Iran diplomacy, who've seized on various pieces that have leaked from the negotiation to press their case that the Obama administration is conceding too much. Republicans and some Democrats believe a deal would be insufficient and unenforceable, allowing Iran to eventually become a nuclear-armed state. And to that end, they've made a series of proposals to undercut or block an agreement, from requiring Senate say-so on a deal to ordering new sanctions against Iran while negotiations are ongoing.

Last week, 47 of the Senate's 54 Republicans signed an open letter to Iran's leaders warning that any nuclear pact they cut with President Barack Obama could expire the day he leaves office. The action prompted fierce criticism from top administration officials, who declared it an unprecedented interference in the president's conduct of U.S. foreign policy.

Appearing on CNN, Senate Majority Leader Mitch McConnell defended the letter Sunday, accusing Democrats of selective outrage and predicting the emergence of a ``very bad'' nuclear deal. Its author, freshman Sen. Tom Cotton of Arkansas, added that he had no regrets, saying the blowback only underscored that Obama wasn't negotiating for ``the hardest deal possible.''

In his interview, Kerry said Tehran ``to its credit'' has entirely lived up to an interim agreement reached in November 2013.

But that understanding was only a stopgap measure, not doing nearly enough to satisfy the long-term concerns of Israel or Iran's Sunni Arab rivals in the Middle East, or the United States. Experts say the combination of limits on Iran's uranium program only gives the world two to three months to react if the country tries to surreptitiously ``break out'' toward nuclear weapons development. The U.S. says it needs at least a year of cushion time, lasting for at least a decade, in a comprehensive agreement.

It's unclear if negotiators will reach that point, putting the United States in a difficult spot. Fearful that Iran could be playing for time, Obama, Kerry and various officials have vowed to walk away from the talks if they show no sign of pointing toward a satisfactory agreement. And they've repeatedly stressed that ``no deal is better than a bad deal.'' But none of them have spelled out what the U.S. strategy for preventing a nuclear-armed Iran would be then.