"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," Ashton said after the parties met in the Austrian capital.
"There is a lot to do, it won't be easy but we have made a good start," Ashton told reporters.
"We have set a timetable of meetings, initially over the next four months with a framework to continue our deliberations," she said.
The discussions between Iran and the United States, China, Russia, Britain, France and Germany aim to transform a landmark but only interim deal agreed in Geneva in November into a long-term agreement.
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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.
The aim of the final deal would be for Iran to retain its civilian nuclear programme, but likely on a reduced scale and with enhanced oversight to ensure a dash for nuclear weapons is all but impossible.
Experts say this would likely involve closing the underground Fordo facility, slashing the number of uranium centrifuges, cutting the stockpile of fissile material, altering a new reactor being built at Arak and tougher UN inspections.