Secretary of State John Kerry and his Iranian counterpart, Mohammad Javad Zarif, resume nuclear talks Switzerland before a March 31 deadline for a political agreement. (Reuters)

Down-to-the-wire negotiations to restrict Iran’s nuclear program and ease sanctions are ready to kick off under the pressure of a self-imposed deadline only two weeks away.

Secretary of State John F. Kerry arrived in Lausanne on Sunday evening and was to meet with Iranian Foreign Minister Mohammad Javad Zarif on Monday. Other members of the U.S. negotiating team, including Undersecretary of State for Political Affairs Wendy Sherman and Energy Secretary Ernest Moniz, arrived earlier and were seen wandering the grounds of a luxury hotel where the discussions will take place in this lakeside city near the border with France.

After more than a decade of talks that gained momentum over the past year and a half, there is a last-chance feel to the latest round. The discussions have been stalled over the pace of sanctions relief, inspections and the size of Iran’s nuclear capacity.

The negotiations with Iran are being conducted by six world powers — the United States, France, Britain, Russia, China and Germany. They have said that they want a framework for an agreement by the end of this month, leaving three months to work out some complex technical details.

Kerry, though, said in a CBS interview that aired Sunday that many of the technical differences have already been resolved.

“By and large, most of the differences now are political decisions that need to be made in order to fulfill the promise of proving to the world that a program is peaceful,” he said in the interview, conducted Saturday in Egypt, where he was attending a regional economic conference.

Kerry added that Tehran, “to its credit, has thus far lived up to every part of the agreement we made over a year ago,” referring to an interim agreement that has stalled parts of Iran’s nuclear program.

The talks are being held against the backdrop of a political firestorm in Washington, after 47 Republicans sent an open letter to Iran’s leaders warning that Congress would not necessarily go along with an agreement and that a future president could overturn it.

[Read: The misguided, condescending letter from Republican senators to Iran]

Kerry said a third extension of the interim deal is unlikely.

“We believe very much that there is not anything that is going to change in April or May or June that suggests that at that time, the decision you can’t make now will be made then,” he said. “If it’s peaceful, let’s get it done. My hope is that in the next days, that will be possible.”

Iran is keen on getting some relief from separate sets of sanctions imposed by the United States, the European Union and the United Nations. Zarif flies to Brussels on Monday to talk with E.U. officials.

Kerry is scheduled to be in Lausanne through Friday. If there is no agreement by then, the parties may break for the Persian New Year, beginning Friday night. If necessary, they could return next week.

In other remarks in the CBS interview, Kerry said the United States is stepping up its diplomatic efforts “in a significant way” to pressure Syrian President Bashar al-Assad into negotiations that could end, or at least decrease, the bloodshed in his country’s civil war.

“We have had conversations with a number of different critical players in this tragedy — and it is a tragedy,” Kerry said, although he did not specify the players. “And we are working very hard with other interested parties to see if we can reignite a diplomatic outcome.”

Kerry said the only solution to the strife in Syria is political, not military.

“But to get the Assad regime to negotiate, we’re going to have to make it clear to him that there is a determination by everybody to seek that political outcome and change his calculation about negotiating. That’s underway right now. And I am convinced that, with the efforts of our allies and others, there will be increased pressure on Assad,” he said.

Eventually, he added, the United States will have to negotiate with the Assad regime. State Department officials said that would not necessarily mean negotiating with Assad himself.

“Our policy has not changed,” said Marie Harf, State’s deputy spokeswoman. “There is no future for a brutal dictator like Assad in Syria, and we remain committed to pursuing all diplomatic avenues to negotiating a political solution. As the president and Secretary Kerry have said, Assad has lost all legitimacy, and his regime’s brutality against the Syrian people has helped violent extremists recruit support.

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