“Today’s negotiations indicated that differences are serious, however, the parties’ determination to settle the differences is also serious,” the source said in Vienna on Monday, after a plenary meeting of foreign ministers of Iran and the Group 5+1 (Russia, China, the US, Britain, France and Germany).

The foreign ministers met twice at Vienna’s Palais Coburg on Monday, the day before their most recent self-imposed deadline for hammering out a deal.

Iran and the Group 5+1 (also known as the P5+1 or E3+3) 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.

They are busy negotiating in Vienna to clinch a comprehensive nuclear deal. Last week, the parties gave themselves an extra week beyond an end-June deadline to nail down details of the long-awaited accord.

Earlier on Monday, a source familiar with the negotiations said the final deadline for the comprehensive accord falls on late Wednesday or the early hours of Thursday, July 9.