"We are continuing to negotiate for the next couple of days. This does not mean we are extending our deadline," Mogherini told reporters in front of Vienna's Palais Cobourg.

She noted that some foreign ministers are going to leave Vienna on Tuesday, but will come back again.

Mogherini, who said will stay in the Austrian capital, announced that the nuclear negotiations have reached the most difficult stage.

Meanwhile, Russia's Foreign Minister Sergei Lavrov said the talks are moving forward, but there is no breakthrough deal yet.

Fewer than 10 issues have remained to be solved, Lavrov explained, adding that "almost everyone" would be leaving Vienna.

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 could fall on late Wednesday or the early hours of Thursday, July 9.