With the March 31 target fast approaching, the top diplomats from the five permanent members of the UN Security Council, Germany and Iran were meeting to try to bridge remaining gaps and hammer out the framework deal that would be the basis for a final accord to be reached by the end of June.
US Secretary of State John Kerry and his Iranian counterpart, Mohammad Javad Zarif, have been meeting in the Swiss town of Lausanne since Thursday in an intense effort to reach a political understanding on terms that would curb Iran's nuclear activities in exchange for sanctions relief.
In addition to sticking points on research and development, differences remain on the timing and scope of sanctions removal, the officials said.
And, in a sign that a deal is unlikely today, Russian Foreign Minister Sergey Lavrov will leave the talks, just a day after arriving, to return to Moscow for previously planned meetings, according to his spokeswoman Maria Zarakhova. Lavrov will return to Lausanne tomorrow if there is a realistic chance for a deal, she said.
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By accepting the restrictions, the Iranians would "live up to their rhetoric that they are not trying to acquire a nuclear weapon," he said in Washington on ABC's "This Week."
The Obama administration says any deal will stretch the time Iran needs to make a nuclear weapon from the present two to three months to at least a year. But critics question that, and say it would be flawed because it keeps Tehran's nuclear technology intact.
"But we do not shut our eyes, and we will continue to act against any threat," he said an allusion to Israeli warnings that it will use force as a last resort against Tehran's nuclear program.