Iran_Nuclear_Negotiators_2014-02-18Nuclear talks between Iran and six world powers went past a deadline and into overtime as negotiators renewed efforts to hammer out an agreement.

Enough progress had been made to warrant the extension past midnight, US 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 yesterday, 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 local time in Lausanne, Switzerland.

The negotiations included foreign ministers from all seven nations at the table – Iran, the US, Russia, China, Britain, France and Germany.

After more than a decade of diplomatic efforts to limit Tehran’s nuclear advances, the talks had already been extended twice, demonstrating the difficulties of reaching an agreement that meets the demands of both sides.

The US and its negotiating partners demand curbs on Iranian nuclear activities that could be used to make weapons, and 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 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,” Mr Earnest said.

President Barack Obama held a video conference last night with Mr Kerry and other members of his national security team, including Vice President Joe Biden.

Officials had hoped to wrap up the current talks by last night with a joint general statement agreeing to start a new phase of negotiations to curb Iran‘s nuclear programme.

The 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.

Such documents would allow the sides to claim 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 P5+1 nations was concluded in November 2013.

President 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 fromIran‘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, Mr Obama can expect stiff opposition at home from members of Congress who want to move forward with new, stiffer Iran sanctions.

Politicians 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 containIran‘s nuclear work and possibly lead Israel to act on threats to use military force to accomplish that goal.

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.

In Israel, Prime Minister Benjamin Netanyahu renewed his 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.

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.