VIENNA, Austria (CNN) - A scheduled meeting of delegations from Iran, France, Russia and the International Atomic Energy Agency was delayed for several hours, but finally began late Tuesday.
It was not clear what caused the delay.
Tuesday's meeting marked the second day of an attempt to strike a deal about the future of Tehran's nuclear program.
Monday, talks among the representatives ended on an optimistic note, with the director-general of the U.N.'s nuclear watchdog agency calling them constructive.
The participants include Iran's envoy to the IAEA, Ali-Asghar Soltaniyeh, and officials from France, Russia and the United States.
Tehran says its nuclear program is intended solely for peaceful purposes. But many in the international community have accused Iran of trying to develop nuclear weapons capability.