Vienna (AFP) - Iran and six world powers begin a final round of nuclear talks in Vienna Tuesday, six days before a deadline for a deal with differences still considerable despite months of negotiations.
"We still have gaps to close and do not yet know if we will be able to do so," a senior US official warned late Monday.
If the sides do strike an accord, it would consign to history a 12-year standoff between the West and the Islamic republic over its nuclear programme by making any Iranian drive to make atomic weapons virtually impossible.
It would also help normalise Iran's relations with the West, improve the lives of ordinary Iranians and give US President Barack Obama a rare foreign policy success.
The United States, China, Russia, Britain, France and Germany (the P5+1) want Iran to scale down and put limits on its nuclear programme.
Iran, which says its nuclear aims are exclusively peaceful, wants painful sanctions lifted and a recognition of its "right" to a peaceful nuclear programme.
US and Iranian negotiations are under domestic pressure not to give too much away, however, while Israel -- widely assumed to be the Middle East's sole nuclear-armed power -- and others in the volatile region are sceptical.
Last November, after moderate Hassan Rouhani became president, an interim deal was struck but the parties missed a July 20 deadline for a lasting accord, giving themselves four more months.
Now, with this new deadline days away, it is "time to finish the job", said chief US negotiator Wendy Sherman.
Iranian Foreign Minister Mohammad Javad Zarif was due to hold a working lunch in the Austrian capital Tuesday with the powers' lead negotiator, former EU foreign policy chief Catherine Ashton.
The first session involving all sides was scheduled in the afternoon. Foreign ministers including US Secretary of State John Kerry were expected later in the week.
At present Iran could use its existing infrastructure to produce enough weapons-grade uranium for one bomb in a few months, although any such "breakout" attempt would be detected.
And Iran wants to ramp up massively the number of enrichment centrifuges in order, it says, to make reactor fuel.
The West wants them slashed, saying Iran has no such need at present, while seeking to extend the "breakout" period to at least a year.
Other thorny issues are the duration of the accord and the pace at which sanctions are lifted, an area where Iranian expectations are "excessive", one Western diplomat said.
"They want everything, all at once and this is not realistic," the diplomat involved in the talks said.
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