LAUSANNE, Switzerland:With stubborn disputes unresolved, nuclear talks between Iran and six world powers went past a self-imposed deadline and into overtime as negotiators renewed efforts to hammer out the outline of an agreement.
Enough progress had been made to warrant the extension past midnight Tuesday, State Department spokeswoman Marie Harf said, although there still were "several difficult issues" to bridge.
Secretary of State John Kerry, who had planned to leave the talks Tuesday, was remaining. And an Iranian negotiator said his team could stay "as long as necessary" to clear the remaining hurdles.
The decision came after six days of marathon efforts to reach a preliminary understanding by midnight Tuesday, drawing in foreign ministers from all seven nations at the table Iran, the United States, Russia, China, Britain, France and Germany.
After more than a decade of diplomatic efforts to limit Tehran's nuclear advances, the present talks already had been extended twice, demonstrating the difficulties of reaching an agreement that meets the demands of both sides.
The U.S. and its negotiating partners demand curbs on Iranian nuclear activities that could be used to make weapons, and they say any agreement must extend the time Tehran would need to produce a weapon from the present several months to at least a year. The Iranians deny such military intentions, but they are negotiating with the aim that a deal will end sanctions on their economy.
In a sign of the confusion surrounding the end of the talks, Russian Foreign Minister Sergey Lavrov claimed there was agreement on all sides. That statement was quickly contradicted by a Western diplomat.
In Washington, White House press secretary Josh Earnest suggested that talks meant to produce an outline that would allow the sides to continue negotiations until the June 30 final deadline had not bridged all gaps. He said the sides were working to produce a text with few specifics, accompanied by documents outlining areas where further talks were needed.
"If we are making progress toward the finish line, then we should keep going," Earnest said. President Barack Obama held a video conference Tuesday night with Kerry and other members of his national security team, including Vice President Joe Biden and Defense Secretary Ash Carter.
Officials had hoped to wrap up the current talks by Tuesday night with that joint general statement agreeing to start a new phase of negotiations to curb Iran's nuclear program. That statement would be accompanied by more detailed documents that would include technical information on understandings of steps required on all sides to resolve outstanding concerns.
Those documents would allow the sides to claim that the new phase of talks would not simply be a continuation of negotiations that have already been twice extended since an interim agreement between Iran and the so-called P5Ư nations was concluded in November 2013.
President Barack Obama and other leaders have said they are not interested in simply a third extension.
The softening of the language from a framework "agreement" to a framework "understanding" appeared due in part to opposition to a two-stage agreement from Iran's supreme leader, Ayatollah Ali Khamenei. Earlier this year, he demanded only one deal that would nail down specifics and not permit the other side to "make things difficult" by giving it wiggle room on interpretations.
But if the parties agree only to a broad framework that leaves key details unresolved, Obama can expect stiff opposition at home from members of Congress who want to move forward with new, stiffer Iran sanctions. Lawmakers had agreed to hold off on such a measure through March while the parties negotiated.
The White House says new sanctions would scuttle further diplomatic efforts to contain Iran's nuclear work and possibly lead Israel to act on threats to use military force to accomplish that goal.
Critics will likely accuse the Obama administration of backing away from promises of a tougher March agreement.
Sen. Tom Cotton, R-Ark., said Tuesday that extending the talks "proves once again that Iran is calling the shots." He said the Obama administration has made "dangerous concessions" to the Iranians over the past week, though he did not specify them.
"The best solution is walk away from the nuclear negotiations now and return to a position of strength," Cotton said in a written statement. He said the U.S. should reinstate existing sanctions that have been suspended, and Congress "should act immediately to impose new sanctions."
In a letter signed by Cotton and 46 other Republican senators in early March, the lawmakers warned Tehran that any nuclear agreement with the Obama administration that lacks congressional approval could be unraveled by future presidents.
Kerry late last year said the focus for March was agreement on "the major elements" of a comprehensive deal that would set a "clear path" for a June deal. If that failed to materialize, "we can revisit how we then want to choose to proceed," he added.
Obstacles remain on several main issues uranium enrichment, where stockpiles of enriched uranium should be stored, limits on Iran's nuclear research and development and the timing and scope of sanctions, among other issues, according to negotiators.
In Israel, Prime Minister Benjamin Netanyahu renewed his severe criticism of the unfolding deal, saying it would leave intact much of Iran's nuclear infrastructure, including underground research facilities, a plutonium reactor and advanced centrifuges capable of enriching uranium.
The U.S. says any final deal will accomplish a goal of stretching the time Iran would need to make a nuclear weapon from several months to a year. But Netanyahu said Washington initially promised "years" to a breakout time.
"In our estimate, it will be reduced to perhaps a year, most likely much less than that," he said.
Kerry and others, including Mohammad Javad Zarif, the Iranian foreign minister, have said the sides have made some progress. Other officials have said Iran is considering demands for further cuts to its uranium enrichment program but pushing back on how long it must limit technology it could use to make atomic arms.
Tehran says it wants to enrich only for energy, science, industry and medicine. But many countries fear Iran could use the technology to make weapons-grade uranium.
