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Iran nuclear talks running out of time

The tense, international negotiations aimed at preventing Iran from developing nuclear weapons are headed down to the wire.

A senior U.S. official on Sunday told reporters in Lausanne, Switzerland, that the talks involving the so-called P5+1 — the United States, United Kingdom, Russia, China, France and Germany — and Iran remain focused but not panicked as a self-imposed March 31 deadline loomed. The official seemed prepared for meetings to last right up until midnight Wednesday.

Negotiators had planned a Sunday afternoon meeting, but the list of attendees wasn't immediately clear.

The major sticking point, the official said, 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.

Related: Will US reach a nuclear deal with Iran?

The March 31 deadline is to reach a basic deal, with another three 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.

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.

American 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