VIENNA (Reuters) - Tehran and world powers were still shy of a breakthrough at nuclear talks on Thursday as foreign ministers flew in to help push for a swift deal and resolve disputes over how sanctions could be lifted and how Iran's compliance would be monitored.
Iran is in talks with the United States and five other powers - Britain, China, France, Germany and Russia - on an agreement under which it would curtail its nuclear program in exchange for relief from economic sanctions.
French Foreign Minister Laurent Fabius said some progress had been made and that he would return to Vienna on Sunday evening in the hope of clinching a final deal to end a 12-year standoff between Iran and the West.
Chinese Foreign Minister Wang Yi said there was a "high possibility" Iran and the six would succeed in reaching an accord in the coming days, though he cautioned that there were still difficult issues to resolve.
"We have confidence that finally the parties concerned will arrive at a fair, balanced and just solution," he said through an interpreter.
He added that they were "faced with some important and sensitive issues which no one can shy away from."
A senior Iranian official told reporters Iran and the six were discussing possible dates when key U.S., European Union and United Nations sanctions would be lifted and limits on Iranian nuclear activities put into effect.
The idea was to have both sides prepare everything so that the implementation of a deal would start on a specific day, he added. However, the official highlighted a major difference between Iran and Western powers when he said Tehran expected key international sanctions would be "terminated".
U.S. and EU officials have spoken of sanctions being suspended rather than ended, so that they can be reimposed if Iranian fails to comply with a deal.
Western powers and their allies suspect Iran is using a civilian nuclear energy program as a cover to develop a nuclear weapons capability. Iran denies this, saying its program is for peaceful purposes.
The negotiators missed a June 30 deadline for a final agreement but have given themselves another week, until July 7.
A deal would be a major policy achievement for both U.S. President Barack Obama and Iranian President Hassan Rouhani. Both face scepticism from powerful hardliners at home in countries that have been enemies since Iranian revolutionaries stormed the U.S. Embassy in Tehran in 1979.
Iranian Foreign Minister Mohammad Javad Zarif and U.S. Secretary of State John Kerry, who have been holding intense talks in Vienna, were joined on Thursday by the foreign ministers of Britain, Germany and China, as well as France.
Britain's Philip Hammond told reporters on arrival: "I don't think we're at any kind of breakthrough moment yet and we will do whatever we need to do to keep the momentum."
Iranian negotiator Majid Takhteravanchi told reporters: "We have reached the final days, but still it is not clear when the last day will be."
Major issues yet to be agreed also include monitoring and verification steps to ensure that Iran does not cheat on any agreement.
The powers want Iran to grant more access to inspectors from the International Atomic Energy Agency, and to answer its questions about previous nuclear work that may have had military purposes. IAEA chief Yukiya Amano was in Tehran on Thursday to meet Iranian officials.
Iran's Supreme Leader Ayatollah Ali Khamenei, who has the power to block a deal, last week ruled out either a long freeze of sensitive nuclear work or opening military sites to inspectors.
A Western diplomat told Reuters: "The positions set out by Khamenei last week make it more difficult to bridge the gaps in the next few days, and there is still work to be done."
In an apparent response to Khamenei, Obama said on Tuesday no deal would be agreed unless it blocked all Iranian pathways to developing a nuclear bomb and ensured a robust monitoring system was in place.
The senior Iranian official in Vienna said Iran would sign up to a special IAEA inspection regime called the Additional Protocol, which would be provisionally implemented at the start of a deal and, eventually, ratified by Iran's parliament.
The Protocol allows IAEA inspectors increased access to sites where they suspect nuclear activity is taking place, but U.S. officials say it is insufficient because it could allow Iran to stall by dragging out negotiations over access requests.
Iran's IAEA ambassador Reza Najafi called Amano’s visit "constructive", according to the state news agency IRNA. The IAEA had no immediate readout.
A senior U.S. official said this week the powers had agreed on a procedure to ensure the IAEA got the access it needed, though there was no suggestion that Iran had accepted it.
Reporting By Louis Charbonneau, John Irish, Parisa Hafezi and Arshad Mohammed; Writing by Arshad Mohammed; Editing by Peter Graff and Kevin Liffey