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What happens if nuclear talks with Iran fail?

LAUSANNE, Switzerland -- As the U.S. helps an Arab coalition carry out bombing runs against Iranian-backed rebels in Yemen, it is trying hard to avoid taking its own military action to destroy Tehran's nuclear program.

That threat of force is the leverage underpinning these negotiations in Lausanne to peacefully persuade Iran to freeze its nuclear development for a decade or more. Yet few diplomats believe that armed confrontation with Iran is imminent should diplomats fail to reach an agreement by the self-imposed end-of-March deadline.

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"If you look at something like military action, you know the president has said all options are on the table, but who wants at this point to undertake another military action in a region that is already on fire in so many places, who wants to sign the U.S. up for that?" State Department spokeswoman Marie Harf told CBS News. "I don't hear a lot of people saying that."

The more likely consequence is another round of punishing financial sanctions on Iran. The trump card Tehran holds is that it could decide to use that pressure as an excuse to charge ahead with the nuclear development that the U.S. believes it had put on hold for the duration of these talks.

That agreement - part of the Joint Plan of Action agreed to by the P5+1 and Iran in 2013 - is scheduled to expire later this June. That gives negotiators a bit of breathing room to decide what's next if they miss Tuesday's deadline.

Yet the recent explosion of violence in Yemen has injected more pressure into these already intense negotiations. U.S. ally Saudi Arabia feels particularly threatened by the reach of Iranian-armed rebels along its border with Yemen, Tehran's arming and financing of Hezbollah, the Islamic Republic's support for the Assad regime in Syria and Shiite militias in Iraq.

Harf said that Iran's destabilizing influence in the region raised the stakes for this diplomatic deal.

"Obviously the Middle East is a more complicated place than it has been in decades," Harf said. "We are facing an incredible set of challenges and an Iran with a nuclear weapon would make all those challenges even worse, and that's why we're working on this."

Negotiators insist that they have never been closer to closing a potentially historic deal with Iran to freeze but not dismantle its nuclear program. Yet over the past 24 hours, diplomats here in Lausanne are signaling that the hurdles in these already intense negotiations have merely gotten higher. U.S. officials describe talks as "tough" and "very serious" yet are reluctant to break away.

"The truth is if we cannot solve this diplomatically, the president and our national security team will fairly soon have to make very serious decisions about what the course of action is, and those decisions are among the most serious the president will ever have to make," Harf told CBS News.

The pressure points are multiplying here as foreign ministers from the P5+1 alliance against Iran fly here to weigh in on the U.S.-led diplomacy.

France's Foreign Minister Laurent Fabius has taken a vocal hardline position in these talks. He told reporters that he would "insist" on monitoring and transparency mechanisms to make sure that Iran does not build an atomic bomb.

Up to this point, Iran has resisted pressure to allow far-reaching inspections and monitoring of all of its potential nuclear sites including underground bunkers, uranium mines and mills, and military bases.

Among the concessions Iran hopes to win is a lift of U.N. sanctions, which are the prime financial leverage used by the P5+1 in an attempt to cut off Iran's ability to purchase and produce atomic weapons. It is not clear if those sticking points can be overcome.

The British, Chinese and Russian foreign ministers are set to arrive on Sunday. The chief EU and German diplomats are already here. All of these parties are part of the P5+1 alliance against Iran and must sign off on any arms control deal.