TEHRAN, Iran — With a first round of nuclear negotiations with world powers ending without agreement, Iranian officials looked ahead Sunday to parallel talks with the U.N.’s nuclear chief amid reminders that concessions by Tehran have limits.

The overall message from Iran closely mirrored the work-in-progress tone of U.S. Secretary of State John Kerry and other powerful envoys, who failed to seal a first-step accord during overtime talks in Geneva but then quickly agreed to try again next week.