A March 31 deadline looms for Kerry and Mohammad Javad Zarif to agree the outlines of an agreement, over the objections of US and Iranian hardliners wary of any deal.
Should they reach an agreement, which is meant to be then finalised by July, it will aim to bury any prospect that Iran might develop nuclear weapons after more than a decade of rising tensions and threats of war over its atomic programme.

On Monday, after Kerry, Zarif and others met for almost five hours in the lakeside city of Lausanne, a senior US official involved said that it remains uncertain whether the deal can be clinched.
"Iran still needs to make some very tough and necessary choices to address the significant concerns that remain about its nuclear programme," the official said on condition of anonymity.
"We're trying to get there. But quite frankly, we still do not know if we will be able to," the official said.
After yesterday's meeting Zarif travelled to and from Brussels - while Kerry went for a bike ride - to meet European foreign ministers and EU foreign policy chief Federica Mogherini.
Political directors from the other five powers involved - Russia, China, Britain, France and Germany - are expected in Lausanne in the coming days, with some expected to arrive on Tuesday.

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