Iran's nuclear chief said on Thursday he was "optimistic" about reaching a ground-breaking nuclear deal, as US and Iranian diplomats launched down-to-the-wire talks chasing a March 31 deadline.

"On the whole I am optimistic," Ali Akbar Salehi, head of the Atomic Energy Organization of Iran, said in an exclusive interview with AFP on the sidelines of the crunch talks which resumed in the Swiss town of Lausanne.

But he warned "there are those who have an interest in more troubles and not dealing with this question (and they) have not been inactive. They are trying to make sure there is no deal."

After 18 months of negotiations around the globe, US Secretary of State John Kerry launched a final round of talks with his Iranian counterpart Mohammad Javad Zarif before Tuesday's deadline with American officials also saying a deal was in sight.

A nuclear deal with Iran ending over a decade of talks is in sight by a March 31 deadline, US officials also said.

"We very much believe that we can get this done by the 31st," a senior State Department official told reporters traveling 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 US - have set the March 31 deadline to agree to a political framework on paring back Iran's nuclear ambitions and stopping it developing an atomic bomb.

The framework is meant to lay out ways to step up international monitoring and reduce Iran's nuclear capability 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 technical details of a comprehensive accord set to be agreed on 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 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.