LAUSANNE, Switzerland - Negotiations over Iran's nuclear program grew frantic Saturday amid signs of discord, with the French and German foreign ministers joining Secretary of State John Kerry in talks with Tehran's top diplomat ahead of an end-of-March deadline for a preliminary deal.
With just four days to go until that target, negotiators in the Swiss town of Lausanne were meeting multiple times in various formats to produce what they hope will be an outline of an agreement that can become the basis for a comprehensive deal to be reached by the end of June.
The State Department said "serious but difficult work" remained for negotiators, with the pace of discussions expected to intensify as "we assess if an understanding is possible."
In another nod to the approaching deadline, Iranian President Hassan Rouhani spoke by phone with German Chancellor Angela Merkel to emphasize the importance of reaching an agreement.
Iranian negotiator Majid Takht-e Ravanchi denied a news report that the sides were close to agreement on a text, and other officials spoke of remaining obstacles, including Iranian resistance to limits on research and development and demands for more speedy and broad relief from international sanctions.
French Foreign Minister Laurent Fabius told reporters as he arrived that the talks have been "long and difficult. We've advanced on certain issues, not yet enough on others."
Iranian Foreign Minister Mohammad Javad Zarif, meanwhile, suggested the blame for any impasses lies with the U.S. and its partners.
"In negotiations, both sides must show flexibility," he wrote on Twitter. "We have and are ready to make a good deal for all. We await our counterparts' readiness."
Iranian nuclear agency chief Ali Akbar Salehi described one or two issues as becoming "twisted." He told Iran's ISNA news agency that the sides were working to resolve the difficulties.
Kerry met early in the day with Zarif, before extended sessions with Fabius and Germany's Frank-Walter Steinmeier. The foreign ministers of Russia, China, and Britain also were expected in Lausanne on Sunday.
"We now are standing at the threshold of a political resolution and a collective political impulse," said Russian deputy Foreign Minister Sergei Ryabkov. "I think these chances [of an agreement] significantly exceed 50 percent."
But diplomats at the talks said their presence does not necessarily mean a deal is almost done.
Steinmeier avoided predictions of an outcome, saying only that a nuclear deal could help ease Mideast tensions.
"The endgame of the long negotiations has begun," he said.
Iran says its nuclear ambitions are purely peaceful; other nations fear it is seeking to develop weapons. Progress has been made on the main issue: The future of Iran's uranium enrichment program. It can produce material for energy, science and medicine but also for the fissile core of a nuclear weapon.
The sides tentatively have agreed that Iran would run no more than 6,000 centrifuges at its main enrichment site for at least 10 years, with slowly easing restrictions over the next five years on that program and others Tehran could use to make a bomb.
The fate of a fortified underground bunker previously used for uranium enrichment also appears closer to resolution.
Still problematic is Iran's research and development program. Tehran would like fewer constraints on developing advanced centrifuges than the U.S. is willing to grant.