EU foreign policy head Federica Mogherini is expected in the Swiss city of Lausanne on Saturday to take part in tough negotiations on Iran's contested nuclear programme, her office said. Mogherini had previously been expected at the talks on Sunday.

She is joining six major powers which are in tortuous negotiations with Iran to try and reach an agreement on the contours of what they hope will be a historic deal by Tuesday.

Since a major diplomatic push to resolve the long-running crisis began in 2013, US Secretary of State John Kerry and Iranian Foreign Minister Javad Zarif have met several times, but have twice missed a deadline to nail down an accord.

The powers want Iran to shrink its nuclear programme in order to make it easy to detect any dash to make a bomb under the guise of its civilian atomic programme.

In return, Iran wants an easing of international sanctions that have excluded the Islamic republic from lucrative oil markets and crippled its economy.

France's top diplomat Laurent Fabius, the most hawkish in the P5+1 group of countries negotiating with Iran since late 2013, was the first European minister to fly in for the crucial talks saying he wanted to reach a "robust deal".

France was "insisting" that any deal included mechanisms to ensure that the Islamic republic, which denies wanting nuclear weapons, complies with its commitments, he said Saturday.

German Foreign Minister Frank-Walter Steinmeier on Saturday also joined the talks, although as late as Friday it was unclear if he would. He said the negotiations were in the "endgame".

"After 10 or almost 12 years of talks with Iran, the endgame of the lengthy talks, so to speak, is beginning here," Steinmeier told reporters before joining his US, French and Iranian counterparts.

"And here, with a view of the Swiss mountains, I'm reminded that as one sees the cross on the summit, the final metres are the most difficult but also the decisive ones," he said.

He added that a successful conclusion of the nuclear talks with Iran "could perhaps bring a bit more calm" to the Middle East.

Asked at the start of their talks Saturday morning whether they were expecting a good day, Kerry replied wryly that "we're expecting an evening today," while Zarif joked "evening, night, midnight, morning."

China's Foreign Minister Wang Yi and Russian counterpart Sergei Lavrov will reportedly fly in on Sunday. Britain's Philip Hammond was on stand-by to come.

It remains unclear what form any deal to emerge from the Lausanne talks would take. Iran's Deputy Foreign Minister Abbas Araghchi told reporters on Saturday morning that "no text has been prepared".

But in an encouraging sign on Saturday afternoon, Iranian Foreign Minister Mohammad Javad Zarif said Saturday he was confident that the outstanding differences in tough nuclear talks could be resolved, saying he believed progress was being made.

Speaking after meeting separately with his German and French counterparts, Zarif told reporters: "We're moving forward. I think we can in fact make the necessary progress to be able to resolve all the issues and start writing them down in a text that will become the final agreement."