World powers and Iran late Sunday began looking at giving themselves more time to reach a nuclear deal, as they struggled to overcome major gaps barely 24 hours before a deadline.
The five permanent members of the UN Security Council and Germany (the P5+1) have been locked in talks with Iran for months to turn an interim deal struck in Geneva that expires on Monday into a lasting accord.
Such an agreement, after a 12-year standoff, is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities, an ambition it hotly denies.
But a last-ditch diplomatic blitz in Vienna this week to secure a deal appeared to be unable to bridge major differences, forcing negotiators to question whether more time is a better option.
"Our focus remains on taking steps forward toward an agreement, but it is only natural that just over 24 hours from the deadline we are discussing a range of options both internally and with our P5+1 partners," a senior US State Department official said.
"An extension is one of those options. It should come as no surprise that we are also engaged in a discussion of the options with the Iranians," added the official.
An Iranian source confirmed to AFP that US Secretary of State John Kerry and Iranian counterpart Mohammad Javad Zarif, meeting for the sixth time since Thursday, "discussed" an extension.
"There is nothing concrete yet," he added.
British Foreign Secretary Philip Hammond said however that the parties would still make a "big push tomorrow (Monday) morning to try and get this across the line".
"Of course if we're not able to do it, we'll then look at where we go from there," he said.
Russian Foreign Minister Sergei Lavrov, a key player who arrived Sunday afternoon, also met both Zarif and Kerry separately, as well as German Foreign Minister Frank-Walter Steinmeier.
Britain and France's ministers had also arrived in Vienna while their Chinese counterpart was due early Monday.
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"What a deal would do is take a big piece of business off the table and perhaps begin a long process in which the relationship not just between Iran and us but the relationship between Iran and the world, and the region, begins to change," US President Barack Obama in an ABC News interview aired Sunday.
- Gaps -
Diplomats on both sides say that the two sides remain far apart on the two crucial points of contention: uranium enrichment and sanctions relief.
Enriching uranium renders it suitable for peaceful purposes like nuclear power but also at high purities for the fissile core of a nuclear weapon.
Tehran wants to massively ramp up the number of enrichment centrifuges -- in order, it says, to make fuel for future reactors -- while the West wants them dramatically reduced.
Iran wants painful UN and Western sanctions that have strangled its vital oil exports lifted, but the powers want to stagger any relief over a long period of time to ensure Iranian compliance with any deal.
- 'Six more months or a year?' -
In view of the hurdles -- and of the dangers posed by a complete collapse -- many experts have long believed that the negotiators would put more time on the clock.
An Iranian source told AFP earlier Sunday, while stressing at that point that adding time was not yet on the table, that the extension "could be for a period of six months or a year."
Another extension -- as happened with an earlier deadline of July 20 -- carries risks of its own including possible fresh US sanctions that could lead Iran to walk away.
Arms Control Association analyst Kelsey Davenport told AFP that getting six more months to a year "would not fly" with the other parties.
Any extension "will have to be very short because there are too many hardliners, particularly in Washington and Tehran, that want to sabotage this deal," she told AFP.