A nuclear deal with Iran capping over a decade of talks is in sight by a March 31 deadline, US officials said, but they remained cautious as top diplomat John Kerry prepared Thursday for down-to-the-wire negotiations.
Kerry will push Iran to agree by Tuesday on the outlines of the long-elusive deal disabling parts of its nuclear infrastructure, in return for an easing of crippling global sanctions, at renewed talks in the Swiss lakeside town of Lausanne.
"We very much believe that we can get this done by the 31st," a senior State Department official told reporters travelling on Kerry's plane.
"We can see a path forward here to get to an agreement, we can see what that path might look like... that doesn't mean we'll get there," the official cautioned.
World powers -- Britain, China, France, Germany, Russia and the United States -- have set the March 31 deadline to agree a political framework on paring back Iran's nuclear ambitions and stopping it developing an atomic bomb.
The framework is meant to lay out specific ways to step up international monitoring and reduce Iran's nuclear capability in order to cut off its pathways to a bomb, in return for an easing of crippling sanctions.
Experts will then have a final few months to grapple with the complex technical details of a comprehensive accord set to be agreed by June 30.
It remains unclear how detailed the framework between Iran and the six powers will be, particularly with the United States and France appearing split on the issue.
A senior European official also said any deal may only be an internal document, a fact sheet -- or not a text at all.
The State Department official said the format of any deal was under negotiation.
"We believe and know that we will have to share as many specific details publicly as we can, with the caveat that the work of doing annexes is very tough work," the US official said.
"March 31 is a real date and it is an important one," the official said, but acknowledged any framework deal would have to be fleshed out by a wealth of detailed, technical annexes by the end of June.
- 'Bad tactic' -
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Kerry is under pressure from a hostile Congress to pin down something concrete after 18 months of intense talks with the Islamic Republic.
But Iran's supreme leader Ayatollah Ali Khamenei has criticised the two-step process.
And France, seen as the most hawkish among the six powers has also expressed misgivings.
France's ambassador to Washington, Gerard Araud, said on Twitter last week that aiming to agree something by March 31 was a "bad tactic" creating pressure to get a deal "at any price".
Iranian Foreign Minister Mohammad Javad Zarif, who will sit down again with Kerry on Thursday, hit out at the group of six for not being able to "coordinate its stance".
The March and late June target dates were set after negotiators failed in November -- for the second time -- to meet a deadline to turn a November 2013 interim deal into a comprehensive accord.
Such a deal could also boost relations between Iran and the West after decades of acrimony, potentially including more cooperation fighting Islamic State militants in Syria and Iraq.
But Iran, which denies wanting the bomb, is loath to dismantle any of its nuclear facilities unless in return the powers dismantle painful UN, US and EU sanctions that have choked its economy.
The six powers counter that they can only be suspended -- not terminated -- over a long period, allowing them to be "snapped back" if Iran violates the deal.
Even before a deal is agreed, critics have been lining up to say that it will not do enough to stop Iran getting the bomb.
These include Israel, the Middle East's sole if undeclared nuclear power, as well as Sunni Muslim Gulf monarchies like powerhouse Saudi Arabia, Shiite Iran's main regional rival.
But the main thorns in President Barack Obama's side have been his Republican opponents, who sparked a furore by warning Iran in an open letter that Obama does not have the power to conclude a durable agreement without their backing.
Republicans are also readying legislation that could impose fresh sanctions on Iran and oblige Obama to get any deal approved by Congress.
Talks are due to start Thursday at 10 am (0900 GMT), with a US official saying the schedule was being kept "incredibly fluid".