Iranian negotiators under Mr. Raisi, the new president — who had criticized his predecessor, Hassan Rouhani, for being too soft — made an important concession to get things rolling by agreeing to work from a draft agreement worked out with Mr. Rouhani’s team, two people familiar with the talks said.

Under that agreement, the U.S. would lift all sanctions related to the nuclear deal (while keeping those for human rights and other issues) and Iran would return to its technical commitments regarding its nuclear program under the old treaty. But critical sticking points remain, such as which sanctions would be lifted and when, and in return for what specific actions by Iran, with an as-yet-to-be determined timeline that would sequence the steps.

Washington’s outlook has been more cautious than Tehran’s. Two senior State Department officials noted some modest progress in the talks, gaining a bit more ground beyond where the negotiations had paused in June. But both officials emphasized, without going into specifics, that major points still needed to be addressed. All the while, patience is wearing thin at the State Department.

“I’m not going to put a time limit on it or give you the number of meters remaining on the runway, except to say, ‘Yes, it is getting very, very, very short,’” Secretary of State Antony J. Blinken told reporters late last year.

While the United States could still offer Iran more sanctions relief, Mr. Vaez from the International Crisis Group said, officials in Tehran have failed to persuade Western negotiators that they are serious about coming back into full compliance with the 2015 accord.

Iran’s foreign minister, Hossein Amir Abdollahian, indicated that Iran may have softened its initial demand for the removal of all sanctions imposed after Mr. Trump exited the deal, including those related to human rights.

But in an interview last week with Al Jazeera, Mr. Amir Abdollahian said that at this stage in Vienna, Iran was pursuing “the removal of sanctions” related only to the original nuclear deal and looking to complete sanctions removal sometime in the future.