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 today.
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.
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.
Also Read
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.
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.