The high-level talks were the first to be conducted since an interim deal was reached in November, Iran's independent Press TV reported.
Wednesday's session was chaired by Iran's Deputy Foreign Minister Abbas Araqchi and Helga Schmid, a top aide to European Union foreign policy chief Catherine Ashton.
The first session of talks Tuesday was presided over by Ashton and Iranian Foreign Minister Mohammad Javad Zari.
The U.N. office in Vienna is hosting the negotiations between Iran and China, Russia, Britain, France, Germany and the United States.
A spokesman for the Atomic Energy Organization of Iran said Wednesday in Tehran the country's supreme leader had expressed opposition to the production and use of nuclear weapons in a 2012 fatwa, or religious degree, reported the Fars News Agency, which has links to the government.
The decree by Ayatollah Seyed Ali Khamenei "strengthens the necessity for non-proliferation, non-storage and non-application" of atomic weapons, said Behrouz Kamalvandi.
He said Khamenei's ruling was "an audible message to the world" that Iran was opposed to weapons of mass destruction.