The talks remained snarled despite the last-minute intervention of Secretary of State John Kerry, who flew to Geneva for the second time in two weeks to try to break the impasse. The Obama administration has been seeking to quickly finalize an agreement in the face of threats by Congress to impose additional economic sanctions on Iran.
The marathon discussions with Iran, which extended into an unscheduled fourth day, were described by Western diplomats as "very difficult" and "intense." Several officials sought to lower expectations that a resolution could be reached before Sunday, when Kerry and the other foreign ministers were due to depart.
Kerry, Zarif and the lead E.U. negotiator, Catherine Ashton, met late Saturday night but the session ended with no announcement of progress. Instead, Iran's deputy foreign minister hardened his country's position.
Although "98 percent" of the deal is done, Iran can accept no agreement that does not recognize what it calls its uranium enrichment rights, Abbas Araghchi told reporters.
"Any agreement without recognizing Iran's right to enrich, practically and verbally, will be unacceptable for Tehran," Aragchi said, according to Reuters.
Araghchi and Zarif have insisted that the deal hinges on international recognition of Iran's right to enrich uranium, a matter of deep national pride.
The proposed deal offered to Iran would reportedly allow limited uranium enrichment, although under tight restrictions and heavy international monitoring. But Western officials have balked at recognizing a legal "right" to uranium enrichment, hoping instead to craft language in the final agreement that acknowledges the right of all countries to pursue nuclear energy for peaceful purposes. Zarif appeared to endorse that approach publicly last week.
The sides also continued to haggle over details of the limited sanctions relief to be offered to Iran in return for scaling back its nuclear program, diplomats said. The relief would reportedly include freeing up a small portion of Iran's overseas currency accounts and easing other trade restrictions.
The most painful sanction, affecting Iran's oil and banking sectors, would remain until the end of the deal's first phase, depending on Iran's willingness to accept permanent curbs on its nuclear program, Western officials said.
Still another obstacle is Iran's partially completed heavy-water reactor in the city of Arak. Western powers are pushing for a freeze on construction of the reactor's core, which could, if completed, give Iran a pathway toward obtaining plutonium for nuclear weapons.
Kerry decided to intervene in the talks after negotiators reportedly made progress in overcoming key obstacles to a deal. Kerry joined Russian Foreign Minister Sergei Lavrov, who arrived in Geneva on Friday, as well as British and French counterparts who traveled to Switzerland early Saturday. Diplomats and technical teams from Iran and six major powers had been meeting privately since Wednesday to resolve a number of sticking points.
State Department officials cautioned that Kerry's decision to attend the talks did not necessarily portend an imminent agreement.
Close U.S. ally Israel opposes the deal as too generous to an enemy it sees as a mortal threat. Israel is not a party to the talks.
The Obama administration has been unable to reassure Israel or another partner, Saudi Arabia, that the arrangement would make the Middle East safer. The interim deal is meant to build confidence and test Iranian follow-through in preparation for a larger agreement to install strict limits on the Iranian nuclear program in exchange for a gradual repeal of sanctions.
Talks were meant to end Friday. But they were extended as foreign ministers joined lower-level negotiators Saturday for what appeared to be final rounds ahead of what would be a historic agreement to rein in nuclear work that the United States and other nations fear is intended to one day produce atomic weapons.
A spokeswoman for Kerry said he would leave Sunday for other diplomatic meetings. Because Kerry is considered an essential player in finalizing the pact, that set an unofficial deadline to either strike a deal or announce that this round came close but not close enough.
Zarif planned to return to Tehran on Sunday. Zarif and Iranian President Hassan Rouhani have pushed for a deal in the past three months, an about-face after years of unproductive talks. The negotiations have yielded the first extended talks between senior U.S. and Iranian diplomats in more than 30 years.