Negotiators discussing the future of Iran’s nuclear program agreed to continue talks seeking a preliminary deal into Wednesday, as stubborn disputes remained unresolved.
A senior State Department official said the negotiators would continue as long as they were making progress. Later, spokeswoman Marie Harf said the decision was made for Secretary of State John Kerry to stay “until Wednesday,” although there still were "several difficult issues" to bridge.
Iranian Foreign Minister Mohammad Javad Zarif sounded upbeat after Tuesday's lengthy talks.
"We have accomplished quite a bit, but people need to get some rest and start over early in the morning. I hope that we can finalize the work Wednesday and hopefully start the process of drafting [the text of a final accord ]," he said.
The White House said President Barack Obama met with his national security team Tuesday night to get an update on the talks.
The two sides hope to reach a preliminary understanding that will allow them to enter a new phase of negotiations aiming at a final deal by June.
Officials ramped up the pace of talks Tuesday to try to craft such a deal, while cautioning that an agreement might come up short of the one envisioned earlier.
Differences remain on such issues as restrictions on uranium enrichment research, how long the deal will last, how quickly sanctions against Iran will be lifted and what to do if Iran violates the terms.
Negotiators in Lausanne, Switzerland, have been working for nearly a week to break an impasse in talks.
Tuesday's meetings started just after 7 a.m. local time, with two sessions with the Iranian team before noon. Kerry told CNN he expected to work "into the night."
"There still remain some difficult issues," Kerry said. "We are working very hard to work those through."
U.S. officials intially said the midnight deadline was firm, but acknowledged flexibility on exactly what negotiators needed to do by then.
A senior State Department official said diplomats were working around the clock, with the negotiators "evaluating where we are throughout the day and making decisions about the best path forward."
The goal of this round is to reach a political understanding on the major elements of an accord to guarantee Iran’s nuclear program is purely peaceful and to end international sanctions against the country
The New York Times reported that "several difficult issues could be deferred for a final agreement in three months."
Kerry has taken a leading role in the talks with his Iranian counterpart, Zarif. But the foreign ministers of Britain, France, Germany, Russia and China also are involved, having been designated by the U.N. Security Council to conduct these negotiations.
Iran and the group of nations have a history of extending their deadlines. They've done so twice since agreeing to an interim nuclear deal in November 2013.
Kelsey Davenport of the Arms Control Association said the basic outline has been clear, if unofficial, for some time.
"Regardless of how much they lay out, a clear picture of the deal is beginning to form," Davenport said. "It clearly will limit Iran’s uranium enrichment program, put in place intrusive monitoring and verification. So all these elements taken together will block Iran’s pathway to the bomb and guard against a covert nuclear weapons program."
Any agreement will also have to lay out a timetable for relief from crippling economic sanctions against Iran, but the full details of that will likely not be known until the final agreement is concluded.
At this stage, the negotiators are in a bind, not only because remaining issues are proving very difficult to solve, but because they are concerned that publishing preliminary conclusions now would only increase criticism.
Still, U.S. officials say they will have to publish some broad points and share more details in secret with members of Congress, some of whom are poised to increase sanctions on Iran if they are not satisfied with the progress.
Analyst Trita Parsi of the National Iranian American Council said preventing congressional critics from acting is a key to keeping the process going.
"Those in the Senate that have been pushing for more sanctions have failed twice before, but they’re not going to fail every time," Parsi said. "At some point, they’re going to succeed. And if they do so, that’s going to be a major headache for the president, and potentially a death knell to the negotiations."
Iran said the key issue was lifting sanctions quickly.
"There will be no agreement if the sanctions issue cannot be resolved," Majid Takhteravanchi, an Iranian negotiator, told Iran's Fars news agency. "This issue is very important for us."
Even supporters of the talks acknowledge that tough compromises will be needed on both sides.
But they warn that the alternative is a return to confrontation, and the risk of military action, if Iran decides to move toward building a nuclear bomb.
On Tuesday, Israeli Prime Minister Benjamin Netanyahu said the deal being hammered out in Lausanne would "pave the way" to a nuclear-armed Tehran.
"The greatest threat to our security and our future was and remains Iran's attempt to be armed with nuclear weapons," Netanyahu said in a speech to parliament.
"It appears the deal being formed in Lausanne will leave Iran with underground facilities, the nuclear reactor in Arak and with advanced centrifuges," he said, adding Israel estimates it would take Iran less than a year to produce the material needed for a nuclear bomb.
Iran has long denied wanting to build nuclear weapons and insists its program is solely for civilian purposes.
Some material for this report came from AP, Reuters and AFP.