Can the landmark 2015 agreement be saved or has time run out to revive the deal?

- What's at stake? -

Talks to rescue the agreement struck by Iran with world powers to limit its nuclear ambitions in return for sanctions relief hang in the balance.

Tehran has been accused of backsliding on progress made earlier in the year, effectively stalling for time, and Washington has said it is preparing unspecified "alternatives" if the talks lead nowhere.

It is unclear exactly what might happen if the talks collapse. Tehran could be expected to move further away from its commitments under the deal while Western allies could be expected to toughen sanctions.

Previous rounds of talks were held between April and June with Britain, China, France, Germany, Iran and Russia -- the remaining parties to the 2015 deal, also known as the Joint Comprehensive Plan of Action (JCPOA).

Washington withdrew from the JCPOA under former president Donald Trump in 2018.

In June, the sides were "70 to 80 percent" towards re-easing sanctions on Iran in return for the limits on its nuclear programme being reimposed, according to a European diplomat.

But when ultraconservative Iranian President Ebrahim Raisi was elected in June, the talks were suspended with the current round eventually beginning on November 29.

This time the delegations are expanded -- with 40 representing Iran alone -- and the talks have moved to the luxury hotel where the original deal was struck in 2015.

The US is participating remotely with European diplomats shuttling between the Iranians and Americans which they say represents a considerable drag on talks.

- What's the mood? -

On Monday night senior diplomats from the "E3" group of Britain, France and Germany warned "time is running out," adding that "without swift progress, in light of Iran's fast-forwarding of its nuclear programme, the JCPOA will very soon become an empty shell".

Iran began exceeding the JCPOA's limits on its nuclear activity in 2019 in retaliation for Trump's withdrawal from the accord and re-imposition of sanctions.

This year Iran has enriched uranium to unprecedented levels, nearing the 90 percent purity needed for a weapon. It insists the programme is entirely peaceful.

The Western participants have complained of "radical" Iranian demands and US Secretary of State Antony Blinken said Tuesday that "Iran is still not engaged in real negotiations".

One diplomat likened the talks to "a dance: one step forward, one step back -- it depends on the day".

The diplomat added that the "discussions are difficult but they are worth having".

Among the key sticking points are precisely which sanctions Washington will lift and the guarantees demanded by Iran to safeguard against future changes of heart from the US.

- What could come next? -

Tehran's chief negotiator Ali Bagheri is seen as belonging to the ultra-conservative camp within Iran and was himself opposed to the 2015 deal.

Nevertheless he has called the talks "constructive".

On Tuesday an Iranian government spokesman said an agreement could be reached "quickly" if the Western states were serious about sanctions lifting and stopped making "threats".

Russia and China have been eager to stress progress but Moscow's ambassador to the UN in Vienna Mikhail Ulyanov has admitted that important technical issues remain to be resolved.

A "political decision" from Iran's Supreme Leader Ali Khamenei was essential for giving the talks fresh impetus, according to one diplomat who spoke to AFP.

Blinken has said the US is "actively engaging with allies and partners on alternatives" in case the talks fail.

Hawks in the US have been pushing for increased economic and diplomatic pressure on Iran, as well as holding out the possibility of military action.

One diplomatic option for the US could be to convene a special meeting of the board of governors of the International Atomic Energy Agency (IAEA) in order to pass a resolution condemning Iran, says Ali Vaez, an Iran expert at the International Crisis Group.

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