Six world powers and Iran missed a Tuesday deadline to reach an outline accord reining in Tehran's nuclear program, extending talks into an extra day, as they edged toward a deal but failed to agree crucial details such as the lifting of U.N. sanctions.
The negotiators ended talks in the Swiss city of Lausanne in the early hours of Wednesday and said they would reconvene later in the day, with Iran and Russia expressing optimism that an initial agreement was within reach.
The preliminary deal is a major milestone toward a final accord, with an end-June deadline, that could end the 12-year-old standoff and reduce the risk of another Middle East war.
With Iran asserting its "nuclear rights" and the United States threatening on Tuesday to abandon the negotiations, the talks have been bogged down on the issues of nuclear research, the lifting of U.N. sanctions and their restoration if Iran breaches the agreement.
Russian Foreign Minister Sergei Lavrov said negotiators had reached a general accord on "all key aspects", according to Russia's TASS news agency, while his Iranian counterpart said a draft agreement could be prepared on Wednesday.
But a diplomat close to the talks denied that such an agreement had been reached, and a French official said Foreign Minister Laurent Fabius was leaving the talks and would return from France when it was "useful".
It was not clear whether Fabius' departure was a sign of a major problem in the talks.
The six powers - the United States, Britain, France, Germany, Russia and China - aim to stop Iran from gaining the capacity to develop a nuclear bomb in exchange for easing international sanctions that are crippling its economy.
Iran says its nuclear program is peaceful.
The U.S. administration of President Barack Obama had committed to meeting a March 31 deadline for the outline political accord.
Negotiators have a tentative agreement on the rough outline of a possible public statement on the progress they have made so far that would also highlight areas of disagreement, diplomats close to the talks said. But they have not agreed on all the key details central to a political framework that would form the basis of a future nuclear agreement, the diplomats said.
Parts of any understanding reached by the parties will likely remain confidential for the foreseeable future.
Failure to agree a detailed preliminary accord would add to skepticism in the Republican-controlled U.S. Congress over an emerging deal. Congress has warned it will consider imposing new sanctions on Iran if there is no agreement this week, giving an added sense of urgency to the talks.
A Democrat, Obama has threatened to veto any sanctions moves by the Republican-controlled Congress.