VIENNA — For Secretary of State John F. Kerry, who has spent years laying the groundwork to secure what he views as a game-changing deal with Iran, the next several days will be crucial as he tries to move tense, slow-paced negotiations toward a historic agreement.
The United States and five other world powers have made strides toward a deal that would curb Iran’s ability to pursue a nuclear weapon in return for lifting economic sanctions. But as talks entered a critical stage after nine days of marathon negotiations, Kerry warned Sunday afternoon that the whole thing could still collapse.
“It’s now time to see whether or not we are able to close an agreement,” Kerry said outside a 19th-century palace where negotiations have been taking place.
“At this point, this negotiation could go either way,” he said, coming to the podium on crutches. “If hard choices get made in the next couple of days, and made quickly, we could get an agreement this week. But if they are not made, we will not.”
He added: “If there’s absolute intransigence . . . we’ll be prepared to walk away.”
The deal would be one of the signature accomplishments of Kerry’s long career, coming nearly four years after he quietly laid the foundation for new talks with Iran while he was still representing Massachusetts in the Senate.
As negotiations stretched into Sunday night, US officials were preparing for a variety of scenarios, including failure. But significant progress has been made in recent days on some of the thorniest aspects, including sanctions relief for Iran and an investigation into its past efforts to develop a nuclear weapon.
Several foreign ministers returned to Vienna on Sunday night to prepare for what they hope is the final round of talks. US officials have warned that they could go past a Tuesday deadline, but negotiators are eager to complete the deal by Thursday. If they don’t, Congress will have 60 days to review the deal instead of 30 days, and Kerry has been trying to avoid giving critics of the deal more time to organize.
“I think he wants a deal with Iran too badly,” Senator Lindsey Graham said in a recent interview. “I would have walked away a long time ago.”
Prime Minister Benjamin Netanyahu of Israel posted several messages on Twitter Sunday, saying the talks “have yielded a collapse, not a breakthrough. The major powers’ concessions are increasing.”
This weekend was spent in a flurry of meetings, interrupted only briefly as the US team paused to celebrate Independence Day.
Kerry has met with his Iranian counterpart, Foreign Minister Javad Zarif, almost every day for several hours. Energy Secretary Ernest Moniz has met more than a dozen times for a combined 24 hours with his counterpart, Ali Akbar Salehi.
The two men — who share strong ties to MIT, where Salehi earned his PhD and Moniz taught — have become so close that Moniz has occasionally joined Salehi in the Iranian dining room for dessert or, as on Friday night, a dinner of kabobs and a Persian stew called fesenjan.
One of the most difficult issues to resolve has been how and when to relieve sanctions that would pump tens of billions of dollars into the Iranian economy.
Iran wants the sanctions lifted immediately, while the United States has insisted that Iran prove it is abiding by the deal first. What appears to be emerging is a phased approach in which the process for lifting sanctions begins at the same time as Iran begins complying with new restrictions on its nuclear program.
Another key development occurred Friday when the head of the International Atomic Energy Agency said he would be able to complete a report by the end of the year on past allegations that Iran has sought to develop a nuclear weapon.
Iran contends that its nuclear program is strictly for peaceful purposes, but international inspectors have said they need broad access and cooperation to verify that.
“They are down to implementation details that can be resolved,” said Daryl G. Kimball executive director of the Arms Control Association, a Washington think tank. “The fundamental issues have been addressed.”
“In the end, this agreement will — and I’m moving from ‘would’ to ‘will’ — be a really historic breakthrough,” he added. “It will resolve one of the biggest proliferation issues we’ve faced in the last 15 years.”
The talks are a result of a long process of incremental engagement that began with personal outreach by Kerry 3½ years ago.
In a trip coordinated with the White House and then-Secretary of State Hillary Rodham Clinton, Kerry traveled to Oman in December 2011 to begin the initial steps to open a dialogue with Iran.
In a bit of irony, when Kerry traveled to Muscat to open that back channel he had two black eyes and a broken nose, from an ice hockey game days before. Now, as he attempts to close the Iran agreement that grew out of that effort, he has been hobbling around with a broken right femur from a bicycle accident.
Kerry is making time for at least one session of physical therapy in his hotel room each day, and he’s now wearing sneakers instead of dress shoes. He’s also been stir crazy at times, trying to escape to nearby parks simply to sit in the sun.
The deal has been all the more complex because it involves six world superpowers on one side of the table — the United States is joined by Britain, China, France, Germany, and Russia — and just Iran on the other. At times it has been difficult to keep a unified front and Iran has used that to its advantage.
“Many people at home and abroad believe he’s been overeager for a deal,” said Robert Einhorn, a former State Department nonproliferation official who served on the US negotiating team with Iran in the past. Einhorn, who has expressed concern with some aspects of the deal, said he thinks Kerry has “kept a clear view of what his requirements were. And he’s fought very hard for those requirements. If they’re in fact met, he will deserve a lot of the credit.”
But just a few hours before Kerry went into his fourth session of the day with Zarif on Sunday night, he sought to lower expectations for what lies ahead.
“Our hope is that we get an agreement that is fair, that gets the job done, and we can hold our heads high and show the world that countries can come together and make things happen,” Kerry said. “But we’re not there yet.”