The two top diplomats held a meeting at Vienna's Palais Coburg hotel to discuss the progress of nuclear talks between Iran and the Group 5+1 (Russia, China, the US, Britain, France and Germany).
They had met on Saturday and Sunday at the same hotel.
The foreign ministers of the G5+1 also started their internal meeting in Vienna at 11:15 a.m. local time.
The negotiating parties missed a June 30 deadline for a final agreement on Tehran's nuclear program, but gave themselves an extra week, until July 7.
Zarif and Kerry have remained in Vienna to try to finalize the long-awaited accord.
Iran and the Group 5+1 (alternatively known as the P5+1 or E3+3) have been negotiating over the past 22 months to end more than a decade of standoff over Tehran's civilian nuclear program once and for all.