The U.S. and other world powers and Iran met Monday in a final push to reach an interim nuclear deal - with one foreign minister saying there had been "some progress and some setbacks" as a deadline loomed in the negotiations.
Talks aimed at preventing Tehran having the capacity to develop a nuclear bomb in exchange for easing international sanctions have been going on for days in the Swiss city of Lausanne.
"I can't rule out that there will be further crises in these negotiations," German Foreign Minister Frank-Walter Steinmeier said told reporters as the talks resumed Monday, adding that there had been "some progress and some setbacks in the last hours."
A senior State Department official denied earlier reports that Iran's refusal to export its atomic fuel was a sticking point that could end prospects of a deal.
Tehran's lead negotiator, Abbas Araqchi, told reporters Sunday that a deal was still "doable." At the same time, he ruled out one of the P5+1's core demands: that Iran send its nuclear stocks out of the country.
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Iran tentatively agreed in November to ship much of its uranium stockpile to Russia, where it would be de-weaponized by converting it into fuel rods for Iran's commercial nuclear power plant.
But Araqchi told reporters Sunday, according to Agence France-Presse: "The export of stocks of enriched uranium is not in our program, and we do not intend sending them abroad. There is no question of sending the stocks abroad."
"The talks are in their final phase and are very difficult," Abbas said. "We are optimistic, the chances of getting a deal are there. But this requires the other side taking the necessary decisions and demonstrating their political will."
Speaking before Araqchi's remarks, the senior U.S. official said another sticking point remains how to get Iran to stop research and development on nuclear activity for the life of the agreement, perhaps as long as 15 years. But there are all sorts of smaller pieces to what has been described as a complex puzzle involving a myriad of interests.
The March 31 deadline is to reach a basic deal, with three more months scheduled in to craft a more detailed agreement. The deadline has been moved twice before, but the Obama administration has said it doesn't want to postpone it again.
The Republican-controlled Congress, meanwhile, is prepared to vote to impose stiffer economic sanctions on Iran in mid-April if a deal isn't reached. Israeli Prime Minister Benjamin Netanyahu has also tried to pressure the president not to concede any ground to Iran.
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Iran has agreed to halt its nuclear work during the talks. That could end if the negotiations fail.
Iran has said that it has no intention of building a nuclear weapon. But it has not been as forthcoming as the United Nations would like about its warhead development.
U.S. and allied negotiators, including France and Germany, have agreed to let Iran perfect centrifuges for medical research at its Fordow facility, but not enrich uranium that could be used for a bomb.
They have also agreed to let Iran keep 6,500 of its 20,000 centrifuges but have put other restraints on fuel and other equipment that they claim would prevent Iran from breaking out.
Any deal would likely involve a phased-in approach to the lifting of sanctions and the limits on certain nuclear work, the U.S. official said.
This story originally appeared on NBC News