Negotiators from Iran and six world powers hunkered down Thursday to a second day of talks aimed towards what could be a historic deal on Tehran's controversial nuclear programme.
Indications of how the talks were progressing in a rainy Vienna were thin on the ground, however. Both sides warned on arrival on Tuesday that the negotiations would be hard.
A spokesman for Catherine Ashton, EU foreign policy chief and the powers' lead negotiator, said only that the first day's discussions were "intensive and useful" and that the "hard work" would continue Thursday.
A US State Department official said that "coordination and experts meetings will resume and continue throughout the day" at a hotel in the Austrian capital.
After three earlier rounds, this time Iran and the five permanent members of the UN Security Council plus Germany aim to start drafting the actual text of what could be a landmark agreement.
Success could help Tehran and Washington normalise relations 35 years after the Islamic revolution toppled the autocratic US-backed Shah but failure could spark conflict and a regional nuclear arms race.
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The parties want to get a deal by July 20, when a November interim deal under which Iran froze certain activities in return for some sanctions relief expires.
This could be extended but time is of the essence with hardliners on both sides -- members of the US Congress and arch-conservatives in Iran -- sceptical of the process and impatient for progress.
The United States, Russia, China, Britain, France and Germany want Iran to radically scale back its nuclear activities in order to make any dash for the bomb virtually impossible and easily detectable.
In return the Islamic republic, which denies wanting atomic weapons, wants the lifting of all UN and Western sanctions, which have caused its economy major problems.
Even though there have been indications of some narrowing of positons, for example on the Arak reactor, both sides are sticking to the mantra that nothing is agreed until everything is agreed.
"Quite frankly, this is very, very, difficult. I would caution people that just because we will be drafting it certainly doesn't mean an agreement is imminent or that we are certain to eventually get to a resolution of these issues," a senior US official said Tuesday.
The talks are tentatively scheduled to last until Friday, with Iranian Foreign Minister Mohammad Javad Zarif telling national media on Tuesday that he expected three more rounds before July 20.