'No breakthrough' seen at Iran nuclear talks
US Secretary of State John Kerry attends a meeting with British Foreign Secretary at a hotel in Vienna, Austria on July 2, 2015. (AFP Photo/Carlos Barria)

World powers negotiating a deal with Iran to curb its nuclear ambitions were grappling for a breakthrough as last-ditch talks dragged into a sixth day Thursday.

With the diplomatic drive playing out both in Vienna and Tehran, ministers held a flurry of meetings as the clock ticked down to a new July 7 deadline.

UN nuclear watchdog chief Yukiya Amano held talks in Tehran with the chairman of Iran's Supreme National Security Council, Ali Shamkhani, and was due to meet President Hassan Rouhani later in the day.

"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 to join US Secretary of State John Kerry.

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 powers -- the United States, Russia, China, Britain, France and Germany -- 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 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."

- '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) unparalleled access to sensitive military sites to verify its claims.

Amano's visit to Tehran at Iran's invitation is aimed at jump-starting a stalled international probe.

"Any deal which ensures the pursuit and progress of the peaceful nuclear industry as well as the unconditional lifting of the unjust and illegal sanctions will be seen as positive," said Shamkhani, quoted by the Iranian news agency IRNA.

Kerry was also expected to lock horns again later with his Iranian counterpart Mohammad Javad Zarif, while EU foreign policy chief Federica Mogherini arrived back in Vienna.

"We are not there yet, but we are moving forward so it's going rather well," Mogherini said.

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

He also denied reports he was returning to Tehran again for further guidance, saying: "No I am here."

French Foreign Minister Laurent Fabius, accompanied by his Chinese counterpart Wang Yi, were also due to arrive in Vienna later in the day.

"We are working very, very hard and we have some very difficult issues," Kerry said Wednesday, a day after the deadline for a deal was effectively extended to July 7.

"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" and Iran's future development of faster nuclear equipment.

One possible compromise on the issue of Iran's past and present nuclear activities might be strictly controlled "managed access" visits to military sites that reassure Iran that IAEA staff are not spying on its facilities, experts say.

Iranian agency ISNA said the country's leaders would offer Amano "suggested solutions" to the deadlock.