Agonising talks towards a grand bargain nuclear deal between Iran and major powers looked set Thursday to drag into the weekend and beyond, with stubborn differences still separating the two sides in Vienna.

There was little indication meanwhile whether the head of the UN atomic watchdog had made any progress in Tehran on one of the main sticking points: a probe into allegations of past nuclear weaponisation work.

French Foreign Minister Laurent Fabius said late Thursday at the end of a sixth day of negotiations involving US Secretary of State John Kerry and an army of other diplomats and experts that some progress had been made.

"Things have advanced but we have not yet reached the end," Fabius told reporters.

"I intend to return Sunday evening. And I hope we will then be in place to move towards a definitive solution which will allow a robust accord," Fabius said.

Iran and the P5+1 group -- the United States, China, Russia, Britain, France and Germany -- have effectively given themselves until Tuesday to reach a deal.

"I don't think we are at any kind of breakthrough moment yet," British Foreign Secretary Philip Hammond said as he arrived in the Austrian capital earlier.

But Hammond stressed: "The work goes on. You're going to see over the next few days ministers coming and ministers going to maintain the momentum of these discussions."

The P5+1 are seeking to finalise a long-sought accord which will put a nuclear bomb beyond Iran's reach, in return for lifting biting sanctions against the Islamic republic.

It would end a 13-year standoff over Iran's suspect nuclear programme, and draw the curtain on almost two years of intense negotiations which resumed in earnest after President Hassan Rouhani came to power in August 2013.

"It is clear that we are not there yet. There are small and big obstacles, and we are working on removing these," said German Foreign Minister Frank-Walter Steinmeier.

"Whether everyone's will and courage will be enough at the end of the day is a question that we can't answer yet."

China's Foreign Minister Wang Yi was more upbeat, saying however that all sides "need to make positive efforts".

"We have confidence that finally the parties concerned will arrive at a fair, balanced and just agreement... I think there is high possibility" of a deal, Wang said.

- 'Not there yet' -

Iran rejects allegations that it has been seeking to develop nuclear arms, and has resisted moves to give the International Atomic Energy Agency (IAEA) unparallelled access to sensitive military sites to verify its claims.

The visit of IAEA head Yukiya Amano to Tehran on Thursday, at Iran's invitation, is aimed at jump-starting a stalled probe.

Late Thursday Tehran time Amano went into a meeting with Rouhani, Iranian television reported. It was unclear what progress if any had been made. An IAEA spokesman declined to comment.

Reza Najafi, Iran's ambassador to the IAEA, told local media only that the negotiations were "constructive".

Asked by journalists waiting outside the elegant Coburg Palace where the talks are being held if he was confident of a deal, Iranian Foreign Minister Mohammad Javad Zarif replied from the balcony of his room: "I have to be hopeful."

"We are working very, very hard and we have some very difficult issues," Kerry had said Wednesday.

"But we believe we're making progress and we're going to continue to work because of that," he told reporters.

Tough issues include the pace and timing of sanctions relief, the mechanism for their "snapback" -- reimposing sanctions swiftly if Tehran violates the deal -- and Iran's future development of faster nuclear equipment.