Experts from Iran and six world powers gather in Vienna on May 12 for the next round of talks meant to result in a nuclear deal to curb Tehran's atomic activities in exchange for sanctions relief.
The technical experts are to be joined later in the week by senior political officials from Iran and the G5+1 group -- the United States, Russia, China, Britain, France, and Germany.
The sides are aiming for a comprehensive agreement by the end of June.
Diplomats say that some progress has been made in drafting an agreement but significant issues remain. The negotiators also are working on technical annexes outlining ways of implementing a deal and timeframes.
The six powers want limits on Tehran's programs that could have a military use.
Tehran denies it is pursuing atomic weapons.