As the deadline has approached, Mr. Kerry and Mohammad Javad Zarif, Iran’s foreign minister, have been joined by the chief diplomats from France, Britain, Germany, China, Russia and the European Union, though Sergey V. Lavrov, the Russian foreign minister, left on Monday.

The United States’ goal is to extend to a year the amount of time, known as the “breakout” time, that Iran would need to produce enough bomb-grade material for a single nuclear weapon. Achieving that objective depends on many factors, including how much nuclear fuel Iran has on hand and how fast it can produce new fuel.

The country has tens of thousands of pounds of uranium in various stages of enrichment, but over the past 18 months it has diluted the portion of its stockpile that was closest to being usable in a weapon.

American officials, however, are looking for a longer-term solution. The simplest approach would be to place much of the fuel out of Iran’s reach. Hopes were raised last year when diplomats believed that Iran would be willing to go along with that approach.

Image Secretary of State John Kerry in Lausanne, Switzerland, on Monday. Credit... Jean-Christophe Bott/European Pressphoto Agency

In November, there were reports that Iran had tentatively agreed to send the fuel to Russia for conversion into fuel rods that could power its only commercial power reactor. Ms. Harf insisted on Monday, after Mr Araqchi declared that Iran would never give up the fuel, that there never had been a tentative agreement and that shipping the fuel out of Iran was not a requirement for an agreement.