Top diplomats from Iran and six world powers geared up on Tuesday for a crucial round of talks aimed at reaching a long-term deal regarding Tehran's nuclear program.
A senior US official said negotiators from the five permanent members of the UN Security Council plus Germany were already in Vienna, preparing for the official start on Wednesday.
In the evening, the powers' principal negotiator, EU foreign policy chief Catherine Ashton, is due to have a working dinner with Iranian Foreign Minister Mohammad Javad Zarif.
This fourth round of talks in the Austrian capital is aimed at transforming an interim deal struck in Geneva in November into a lasting accord before a July 20 deadline.
The six powers want Iran to reduce the scope of its nuclear program in order to make any attempt to make a nuclear weapon practically impossible and easily detectable.
In return the Islamic republic, which claims its nuclear program is peaceful, wants all UN and Western sanctions lifted. Iran is gradually winning access to $4.2 billion of its oil revenues frozen abroad and some other sanctions relief.
"If the odds of the talks collapsing are high, the stakes of failure are higher," Ali Vaez, Iran analyst at the International Crisis Group, told AFP. "Time is of the essence."
"We hope the leadership in Tehran has given the entire delegation ...instructions making it possible to move forward," Moscow's negotiator Sergei Ryabkov told the Voice of Russia.
Problems remain in the way of a deal
Iran has not convinced everyone with its professed possession of a peaceful nuclear program, a doubt that was raised on Sunday when Iran's Supreme Leader, Ayatollah Ali Khamenei, said Western expectations that Iran limit its missile program were "stupid and idiotic." Similarly in January, Khamenei stated that negotiations with the US were a tactic to gain time.
White House spokesman Jay Carney said Monday that the last three rounds were to "understand each other's positions" and that drafting a deal "presents obviously challenges."
One major issue, the Arak reactor, has started to be addressed as UN experts visited the heavy water plant for the first time in two years on December 8, when all of the International Atomic Energy Agency's (IAEA) "technical objectives" were reportedly met.
But others, most notably uranium enrichment and the sequence of sanctions relief "could be harder to bridge," Kelsey Davenport from the Arms Control Association told AFP.
Enriching uranium - increasing the proportion of a fissile isotope using supersonic spinning machines called centrifuges - can be used for peaceful uses, but at high purities it can be used in a nuclear bomb.
Iran already has enough of low-enriched material for several bombs if it decided to "break out" and use its 20,000 centrifuges to enrich this stockpile to weapons-grade.
"Discussions on enrichment are and will be difficult," French Foreign Minister Laurent Fabius told AJC Global Jewish Advocacy, on Monday.
Waiting for an explanation about detonators
Other hurdles include Iran's development of ballistic missiles and its answers to long-standing questions from the IAEA about "possible military dimensions" to its nuclear work in the past.
Some progress was made last year when Iran promised to clarify its need for Exploding Bridge Wire (EBW) detonators, which could theoretically be used in a bomb but which also have other applications.
According to diplomats in Vienna, Iran has yet to convince the IAEA on the detonators issue - which is only the first step to resolving it - ahead of a Thursday deadline.
"If things were on track, the agency and Iran would have agreed, or would be on the cusp of agreeing, the next round of measures," one Vienna diplomat said. "It would seem at the moment that that hasn't been arrived at yet, which is disappointing if that is the case."