GENEVA -- As Secretary of State John Kerry and foreign ministers from other world powers sought to work out an interim agreement to constrain Iran's nuclear program, the Iranian government's insistence on formal recognition of its "right" to enrich uranium emerged as a major obstacle, diplomats said Sunday.
In long hours of closed-door discussions, Western and Iranian negotiators haggled over the language of a possible agreement. Toward the end of a marathon session, some diplomats believed that only a handful of words appeared to separate the two sides.
But the dispute over enrichment rights, among other differences, meant that the talks ended not with the breakthrough many had hoped for, but with only a promise that lower-level negotiators would meet in Geneva in 10 days for more discussions.
Many reports have ascribed the failure of the talks to France's insistence that any agreement put tight restrictions on a heavy-water plant that Iran is building, which can produce plutonium.
But while France took a harder line than its partners on some issues, a senior U.S. official said it was the Iranian delegation that balked at completing an interim agreement, saying that it had to engage in additional consultations in Tehran before proceeding further.
A senior U.S. official who briefed Israeli reporters and experts in Jerusalem on Sunday said the six world powers in the talks had approved a working document and presented it to the Iranians, according to Herb Keinon of The Jerusalem Post, who attended the briefing.
"It was too tough for them," Mr. Keinon quoted the U.S. official as saying of the Iranians. "They have to go back home, talk to their government and come back."
The failure to achieve a breakthrough in Geneva followed a week in which the Iranians had raised the expectations of a possible one, perhaps calculating that this would add to the pressure on Western nations to make concessions.
Both Mr. Kerry and his Iranian counterpart sought to put the best face on the deflating outcome.
"We are all on the same wavelength, and that gives us the impetus to go forward when we meet again," Iran's Foreign Minister Mohammad Javad Zarif told reporters after the talks ended.
Mr. Kerry had a similar message: "There's no question in my mind that we are closer now, as we leave Geneva, than we were when we came, and that with good work and good faith over the course of the next weeks, we can in fact secure our goal."
Still, the failure to conclude an accord gave an opening for critics in Congress, who have vowed to push for tougher sanctions, and in Israel and the conservative Arab Persian Gulf monarchies to mobilize opposition to an agreement.
Sen. Bob Corker of Tennessee, a senior Republican on the Foreign Relations Committee, accused the Obama administration of "dealing away our leverage" in an appearance on the NBC program "Meet the Press."
Speaking to a large gathering of American Jewish leaders Sunday evening, Israel's Prime Minister Benjamin Netanyahu warned that his country would not be the only target of an Iranian nuclear weapon.
"Coming to a theater near you -- you want that?" Mr. Netanyahu asked. "Well, do something about that!"
Defending his negotiating strategy, Mr. Kerry insisted Sunday that the agreement to freeze Iran's nuclear program that he was seeking would be in Israel's interest. "We are not blind, and I don't think we are stupid," Mr. Kerry said on "Meet the Press."
"I think we have a pretty strong sense of how to measure whether or not we are acting in the interests of our country and of the globe," he added, "and particularly of our allies, like Israel and the [Persian] Gulf states, and others in the region."
At the heart of the debate is the Obama administration's two-part strategy, which calls for an interim agreement to temporarily freeze Iran's nuclear efforts for six months so that diplomats have time to try to negotiate a more comprehensive accord.
Iran has asserted repeatedly that it has the right to enrich uranium, a necessary step in producing nuclear fuel both for power plants and, at a much higher level, for weapons. The issue appears central to Tehran's insistence that any talks on initial constraints, like the talks in Geneva, also acknowledge an "end state" for Iran's nuclear program.
The Obama administration is prepared to allow Iran to enrich uranium to the low level of 3.5 percent as part of an interim agreement, as long as Iran agreed to other constraints on its nuclear activity.
But the administration is not prepared to acknowledge at this point that Iran has a "right" to enrich, apparently calculating that any enrichment that Iran might be allowed under a comprehensive accord would be tied to its willingness to agree to strict monitoring and limits on its program.
"The United States does not believe there is an inherent right to enrichment, and we have said that repeatedly to Iran," a senior administration official said before the latest round of talks in Geneva.
Marie Harf, deputy State Department spokesman, declined to comment Sunday on developments in the talks.
Whether the differences with Iran over enrichment can be finessed somehow in the next round of talks is unclear.