Secretary of State John Kerry said yesterday the United States and its negotiating partners "will not rush, and we will not be rushed" into finalising a nuclear deal with Iran, but warned they would abandon talks soon if Iran does not make the "tough decisions" needed for an agreement.

"This is not open-ended," he said after walking on crutches to a podium outside the Coburg Palace hotel in Vienna where the talks are being held.

"President [Barack] Obama made it very clear to me last night we can't wait forever for the decision to be made," Kerry said. "We know that. If the tough decisions don't get made, we are absolutely prepared to call an end to this process."

But Kerry didn't specify how much longer he was willing to remain in Vienna in search of a deal that would place curbs on Iran's nuclear programme in return for sanctions relief.

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Kerry spoke at the end of a day of back-to-back negotiations.

Shortly before Kerry spoke, French Foreign Minister Laurent Fabius approached reporters and said the negotiations would continue into the night in an effort to resolve the remaining differences between Iran and six world powers, known as the P5+1 - the US, France, Britain, Russia, China and Germany.

Fabius compared the negotiations to the end of a marathon.

"There are difficult points that remain, but things are all the same going in the right direction," he said, adding, "I hope we will be able to complete the metres that need to be run."

This round of talks, which began a fortnight ago, has been extended twice as has the interim agreement.

The admission that unresolved issues were still blocking a final deal came one day after Obama held a secure video conference with Kerry and the State Department team.

In the call, Obama "provided guidance related to our ongoing efforts to achieve a good deal between the P5+1 and Iran that meets our requirements", according to State Department officials.

Diplomats from the world powers met on Thursday for a little over an hour, as some of the foreign ministers who had left the Austrian capital on Wednesday returned to join the talks.

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Kerry followed that up with a series of separate meetings with Iranian Foreign Minister Mohammad Javad Zarif and their counterparts from Britain and France.

Kerry followed that up with a series of separate meetings with Iranian Foreign Minister Mohammad Javad Zarif and their counterparts from Britain and France. Photo / Supplied
Kerry followed that up with a series of separate meetings with Iranian Foreign Minister Mohammad Javad Zarif and their counterparts from Britain and France. Photo / Supplied

Separately, Energy Secretary Ernest Moniz and Ali Akbar Salehi, the head of Iran's Atomic Energy Organisation, held a meeting, a by-now routine daily discussion between the two nuclear physicists who are charged with ironing out the technical details in any agreement.

Abbas Araghchi, a Deputy Foreign Minister taking part in the negotiations, told Iranian TV that "only a handful" of key issues remained, including the timetable under which Iran would allow international inspectors to visit nuclear sites while the United Nations, the US and the European Union prepare to lift sanctions.

He also said the two sides were still discussing a key Iranian demand, the lifting of a UN embargo on conventional arms sales to Iran. The US has opposed dropping that ban.

Russia, which hopes to sell arms to Iran, has come out in support of lifting the arms embargo but under the framework agreement settled three months ago, the arms embargo would stay in place.