“We very much believe we can get this done by the 31st,” said the official, who was traveling on Secretary of State John Kerry’s plane to Switzerland. “We see a path to do that.”

Still, the official emphasized that an agreement was not assured, adding that the talks could yet be stymied by an array of complex issues.

Mr. Kerry, who arrived shortly before midnight, plans to resume his talks with Mohammad Javad Zarif, Iran’s foreign minister, on Thursday morning in Lausanne.

Those discussions will build on lower-level negotiations that took place here on Wednesday between Wendy Sherman, an under secretary of state, and Abbas Araghchi, Iran’s deputy foreign minister.

If the negotiations proceed as the Americans hope, foreign ministers from the other world powers involved in the talks will join Mr. Kerry over the next several days to seal the agreement. The six world powers that are negotiating with Iran are Britain, China, France, Germany, Russia and the United States.

In a speech in London on Wednesday, Philip Hammond, the British foreign secretary, also suggested an accord was within reach.

“Agreement is deliverable if we continue on this track,” said Mr. Hammond, who asserted that the failure to reach a deal would mean “a fundamentally more unstable Middle East.”

The agreement Mr. Kerry is seeking this month would outline the main parameters of an accord limiting Iran’s nuclear program. That would set the stage for negotiators to fill in the details and complete a comprehensive accord by the end of June.

Iran has been reluctant to sign an initial agreement this month. Iran’s supreme leader has charged that the United States might renege on the terms before a final accord is reached. And Iran’s leaders might be concerned that signing an initial agreement could expose them to criticism from hard-liners at home before a comprehensive agreement codified the sanctions relief Iran is seeking.

As a result, the form of an initial agreement that might be concluded here over the coming days is still uncertain. The accord might take the form of a general understanding with Iran that American officials would try to supplement by providing a more detailed accounting for Congress.

“We do not know what form this will take if we can get there at the end of March,” said the State Department official, who could not be identified under the protocol for briefing reporters.

“We believe and know that we will have to share as many specific details publicly as we can,” the official added.

During the previous negotiating round, differences emerged over what limits would be placed on the research and development of new types of centrifuges, how long an accord should last and the pace at which the economic sanctions that have been imposed on Iran might be suspended or removed.
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Continue reading the main story

In addition to the United States’ differences with Iran, there have also been disputes between American and French officials over the wisdom of setting a deadline of the end of March for an initial agreement.

Obama administration officials have argued that there is no point in deferring tough decisions and that agreement on the “framework” will enable the White House to resist a congressional push for additional sanctions. But French officials have said that the United States and its negotiating partners could forfeit some of their leverage with Iran by rushing to meet a self-imposed deadline.

Mr. Kerry met Saturday in London with his counterparts from Britain, France and Germany to try to settle on a negotiating strategy to bridge the remaining gaps with the Iranians.

The State Department official said that while the United States and its negotiating partners had a “common approach,” they were still discussing “the best tactical way to address some of this in the room with Iran.”

Energy Secretary Ernest J. Moniz has been a key member of the negotiating team over the past several rounds. On Tuesday, Mr. Moniz held a videoconference with his British, French and German counterparts on the complex nuclear issues in the talks.

If no agreement is reached by the end of March, much of Iran’s nuclear program will still be frozen by a temporary agreement the United States reached with Iran in 2013. The limits of that interim accord will be in effect through June.

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