“We are making efforts to resolve some remaining differences,” Zarif told reporters in the Austrian capital of Vienna late on Sunday.
Regarding some issues, differences remain, the Iranian chief negotiator said, adding that "the outcome is still unknown."
Iran and the Group 5+1 (Russia, China, the US, Britain, France, and Germany) have been negotiating over the past 22 months to end more than a decade of impasse over Tehran's civilian nuclear program once and for all.
The two sides are busy negotiating in Vienna to clinch a comprehensive nuclear deal. The parties on Thursday gave themselves an extra week beyond an end-June deadline to nail down details of the long-awaited accord.
The long-awaited deal is expected to put an end to the anti-Iran sanctions that have obstructed Iran's trade with the world.