Claiming enough progress had been made to warrant an extension after six days of intense bartering and eager to avoid a collapse in the discussions, U.S. Secretary of State John Kerry and his British and German counterparts huddled in the Swiss town of Lausanne to continue a marathon effort to bridge still significant gaps and hammer out details of a framework accord.

The foreign ministers of China, France and Russia all departed Lausanne overnight, although the significance of their absence, particularly when the broader group meets with Iranian Foreign Minister Mohammed Javad Zarif, was not clear.

If they succeed, those understandings would form the basis for a comprehensive agreement to be reached by the end of June.

After the talks last broke in the early hours of Wednesday, Zarif said solutions to many of the problems had been found and that documents attesting to that would soon be drafted. Other officials were more skeptical.

Asked how high the chances of success were, German Foreign Minister Frank-Walter Steinmeier said: "I cannot say." And British Foreign Secretary Philip Hammond said Iran might still not be ready to accept what is on the table.

"I'm optimistic that we will make further progress this morning but it does mean the Iranians being willing to meet us where there are still issues to deal with," Hammond told British reporters. "Fingers crossed and we'll hope to get there during the course of the day."

Although the Chinese, French and Russian ministers left their deputies in charge, Kerry postponed his planned Tuesday departure to stay in Lausanne, and an Iranian negotiator said his team would stay "as long as necessary" to clear the remaining hurdles.

Officials say their intention is to produce a joint statement outlining general political commitments to resolving concerns about Iran's nuclear program in exchange for sanctions relief, and their intention to begin a new phase of negotiations to get to that point. In addition, they are trying to fashion other documents that would lay out in more detail the steps they must take by June 30 to meet those goals.

The additional documents would allow the sides to make the case that the next round of talks will not simply be a continuation of negotiations that have already been twice extended since an interim agreement between Iran, the United States, Russia, China, Britain, France and Germany was concluded in November 2013. President Barack Obama and other leaders, including Iran's, have said they are not interested in a third extension.

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.

And despite the progress that diplomats said merited the extension of talks into Wednesday, officials said the differences notably included issues over uranium enrichment, the status of Iran's enriched uranium stockpiles, limits on Iran's nuclear research and development, and the timing and scope of sanctions relief.

The U.S. and its negotiating partners are demanding 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. (**)

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