Washington: US Secretary of State John Kerry left Washington on Wednesday for a date with history, hoping to seal a deal reining in Iran`s nuclear ambitions for years to come.
After months of closed-door negotiations, Kerry and his team was headed once again for talks in Switzerland with Iranian Foreign Minister Mohammad Javad Zarif.
The two nations broke diplomatic ties more than three decades ago during a 444-day hostage taking in the American embassy in Tehran and have remained foes ever since.
But now, negotiators from six world powers hope to meet a March 31 deadline for a landmark political agreement Iran to stop the Islamic Republic from developing a nuclear bomb.
In return, Tehran wants a labyrinth of crippling economic sanctions to be eased or lifted, to free up billions of dollars frozen in bank accounts and to regain access to lucrative oil markets and global trade.
Any deal that brings Iran in from the cold will face opposition from key US allies Israel and Saudi Arabia and from hawks in the US Congress who see it as a naive capitulation.
But Kerry will mount a robust defense, telling an audience of senior US diplomats before he left: "Anybody standing up in opposition to this has an obligation to stand up and put a viable realistic alternative on the table.
"And I have yet to see anybody do that"
An accord would cap more than a decade of painstaking negotiations which, after protracted delays and hurdles, have gathered fresh impetus since the 2013 election of Iran`s President Hassan Rouhani.
It would also mark a legacy-making foreign policy coup in the final years of Barack Obama`s presidency, and a personal triumph for veteran politician and diplomat, Kerry.
State Department spokeswoman Jen Psaki told CNN Wednesday that "it`s a key week" for the negotiators who aim to increase the so-called "breakout time" for Iran to produce a nuclear weapon to about a year.
"The fact is, without these negotiations Iran was moving towards a nuclear weapon," Psaki said, saying opponents of the deal were not offering any other solutions.
"What we`re doing here is extending the period of time giving us more time, giving the international community more time to track and watch what they`re doing."As the clock has ticked down to Tuesday`s deadline, simmering tensions with opponents of a deal have reached boiling point.
And Obama`s Republican foes in Congress are lining up to vote on new Iran sanctions next month, should diplomacy fail.
Washington has also found itself at odds with allies like France, wary that Obama`s administration may be making too many concessions in its haste to reach an agreement.
At best, opponents argue the deal will only delay Iranian nuclear ambitions for a decade or so. At worst, they say, the country`s leaders will be able to covertly move ahead with developing a nuclear weapon.
Kerry`s return to Lausanne to resume the talks with Zarif on Thursday will only fuel speculation that now an agreement is within sight in the coming days.
Other foreign ministers from the so-called P5+1 group -- Britain, China, France, Germany, Russia and the United States -- are reportedly expected to arrive in the lakeside Swiss town over the weekend.
The complex deal on the table would likely involve Iran reducing its nuclear activities, allowing strict, and perhaps even unannounced UN inspections, while mothballing sophisticated equipment needed to enrich uranium.
Iran has always denied seeking a nuclear weapon, and says its atomic energy program is purely for civilian purposes.