US Under Secretary Wendy Sherman and Tehran's Deputy Foreign Minister Abbas Araqchi will resume talks about curbing Iran's nuclear program later on Thursday, Iranian media said.
The latest round of nuclear diplomacy, being held in a luxury hotel in Austria's capital, kicked off with a bilateral meeting between Iran and the European Union on Wednesday.
Iran's nuclear negotiator Hamid Baidinejad told Iranian state television that "drafting the final deal has started," declining to give further details. Months ago both sides had already announced that a final draft had been started.
Iran and the other countries in the talks – the United States, China, France, Russia, Britain and Germany – reached a tentative deal on April 2 and now aim to finalise the details by self-imposed end-June deadline.
Under the framework agreement, Iran agreed to slash the number of uranium enrichment centrifuges it operates and would allow more intrusive inspections by the International Atomic Energy Agency (IAEA) in exchange for sanctions relief.
The diplomatic push needs to iron out details about the timing of sanctions relief, the future of Iran's atomic research and development programme, the exact nature of the IAEA's monitoring regime and what kind of uranium stockpile Tehran will be allowed to keep under any final deal.
The timing of sanctions relief was top of the agenda at Wednesday's talks.
Iran says economic sanctions must be lifted as soon as any final deal is signed, while the United States wants a gradual lifting of restrictions.
Top envoys from other world powers will join the negotiations later this week.