The gruelling diplomatic marathon towards a historic deal putting an Iranian nuclear bomb out of reach entered its final furlong on Tuesday with one week left for Tehran and six major powers to finalise the accord.
Whether they will manage to nail down the agreement by the June 30 deadline is unclear, however, with both sides complaining of differences and saying more time may be needed -- albeit only a few more days.
On Monday Iran's Mohammad Javad Zarif, who is expected in Vienna in the coming days with other foreign ministers, said in Luxembourg that "all sides should avoid excessive demands."
"There is the possibility that we can finish this by the deadline or a few days after the deadline," Zarif said as he met his British, French and German counterparts, saying there was sufficient "political commitment."
British Foreign Secretary Philip Hammond urged "more flexibility" from Tehran, while Iran's lead negotiator Abbas Araghchi said "progress hasn't been what we expected."
In April, Iran and the "P5+1" -- the United States, China, Russia, Britain, France and Germany -- agreed the main outlines of the deal after a bruising rollercoaster round of talks in Lausanne, Switzerland.
After two missed deadlines in July and then November last year, this built on an interim deal struck in Geneva in November 2013 after Iranian President Hassan Rouhani was elected.
According to the Lausanne framework, Iran will downsize its nuclear activities, slashing the number of centrifuges enriching uranium, which can be used in nuclear power but also when highly purified for a bomb.
The powers hope this will ensure Iran would need at least a year -- compared with a few months in 2013 -- to produce a bomb's worth of material. Tight UN inspections would give ample notice of any such "breakout."
In return, UN and Western sanctions that have caused Iran major economic pain would be progressively lifted, although the six powers insist they can be easily "snapped back" if Tehran violates the accord.
After 12 years of rising tensions, Iran denies seeking atomic weapons, saying its programme is for peaceful purposes such as meeting, through nuclear power, the energy needs of its almost 78 million people.
After the Lausanne breakthrough, US President Barack Obama hailed the "historic understanding" and said that if completed, the deal would "make our country, our allies and our world safer."
There were celebrations on the streets of Tehran and Rouhani promised on national television that the accord would open a "new page" in Iran's international relations.
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- Devil in the detail -
Since April, armies of diplomats and experts have been attempting to turn the one-page, 505-word joint Lausanne statement into a beast of a final document which including several appendices will be 40-50 pages long.
"Each word of this instrument is being discussed and sometimes quarrelled on," Deputy Foreign Minister Araghchi said earlier this month.
It will be a highly complex accord, setting out an exact timetable of sanctions relief and reciprocal steps by Iran as well as a mechanism for handling possible violations by either side.
A particular sticking point is thought to be the issue of closer inspections by the UN atomic watchdog, potentially including military sites to probe past -- and any future -- suspicious activity.
"A robust agreement is one which includes an extensive verification element, including if necessary visits to military sites," French Foreign Minister Laurent Fabius said Sunday.
However this is anathema to the Islamic republic. In May supreme leader Ayatollah Ali Khamenei said Tehran "will not allow any inspections of military sites by foreigners."
Other tricky issues include how UN sanctions might be re-applied, the reduction of Iran's uranium stockpile and its future research and development into new types of centrifuges.
Araghchi and senior EU diplomat Helga Schmid were due to meet in Vienna later Tuesday, the EU said, and will be joined by other senior figures later this week including US Undersecretary of State Wendy Sherman.
Zarif, US Secretary of State John Kerry, who broke his leg cycling during a break from talks on May 31, and the other foreign ministers were expected as the deadline approaches.
"The political will to reach a good agreement is there and many of the difficult political decisions have already been made," said Arms Control Association analyst Kelsey Davenport.
"It is certainly possible to wrap up the remaining issues in the eleventh hour and reach an agreement within a few days of June 30," she told AFP.