Iran and world powers have agreed to a timetable and framework agreement for working out a lasting nuclear accord, and will meet again on March 17.
"We had three very productive days during which we have identified all the issues we need to address to reach a comprehensive and final agreement," EU foreign policy chief Catherine Ashton said on Thursday after talks in the Austrian capital.
"There is a lot to do," she told reporters. "It won't be easy but we have made a good start.
"We have set a timetable of meetings - initially over the next four months with a framework to continue our deliberations.
"Technical experts will meet in early March and we will reconvene for the next E3+3 political directors' meeting led by ... (Iranian Foreign Minister Mohammad Javad) Zarif and myself on the 17th of March" in Vienna.
The discussions between Iran and the United States, China, Russia, Britain, France and Germany aim to transform a landmark but only interim deal in Geneva in November into a long-term agreement.
Under the November deal, which came into force on January 20, Iran froze certain nuclear activities for six months in exchange for minor relief from sanctions and a promise of no new sanctions.
