"We very much believe that we can get this done by (March) 31st," said a senior State Department official travelling on Kerry's plane to the Swiss lakeside city of Lausanne.
"We can see a path forward here to get to an agreement, we can see what that path might look like," the official told reporters, cautioning however that this "doesn't mean we'll get there".
Six world powers negotiating with Iran since late 2013 want Iran to disable parts of its nuclear infrastructure in order to put an atomic bomb out of reach and end a 12-year standoff.
After missing two deadlines in 2014 to turn a interim accord struck in November 2013 into a lasting deal, the parties set March 31st for a "framework" agreement with a full pact to be agreed by July.
But 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 what, if anything, is agreed by March 31st was still under discussion but that it "needs to address in some way all of the major elements of a final agreement".
"We have always said it needs to have specifics. We will need to communicate as many specifics as possible to the public in some form or fashion," the official said.
US President Barack Obama's administration is under severe pressure from a hostile Congress to return from Lausanne with something concrete to show from his 18 months of talks with the Islamic Republic.
The White House said Wednesday that Washington expects "tangible, specific commitments . . . by the Iranians".
But Iran's supreme leader Ayatollah Ali Khamenei has criticized the two-step process, with the New York Times reporting that Tehran prefers a more general statement of understanding.
France, seen as the most hawkish of the P5+1 group also comprising Russia, China, Britain and Germany as well as the United States, has also expressed misgivings.
France's ambassador to Washington, Gerard Araud, said on Twitter last week that aiming to agree something by March 31st was a "bad tactic" creating pressure to get a deal "at any price".
Iranian Foreign Minister Mohammad Javad Zarif, who is also under pressure from hardliners at home to deliver, on Wednesday hit out at the P5+1 for failing to "coordinate its stance".
Britain's Philip Hammond, who like other foreign ministers is on stand-by to come to Lausanne, warned of the consequences of failure.
"No deal means no restrictions on (Iran's nuclear programme) . . . It means a fundamentally more unstable Middle East, with the prospect of a nuclear arms race in the region," Hammond said in a speech.
The mooted deal could 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.
And 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 Sunni Muslim Gulf monarchies like powerhouse Saudi Arabia, Shiite Iran's main regional rival, and Israel, the Middle East's sole if undeclared nuclear power and a close US ally.
"We will continue to act to prevent the emerging agreement with Iran — an agreement which endangers us, our neighbours and the world," newly re-elected Israeli Prime Minister Benjamin Netanyahu said Wednesday.
But the main thorns in 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 oblige Obama to get any deal approved by Congress and impose fresh sanctions on Iran — something which would may well torpedo the entire process if passed.