"We've made substantial progress in every area, but this work is highly technical and high stakes for all of the countries involved," Marie Harf, the spokeswoman for the US delegation, said in a statement, according to Reuters.
"We're frankly more concerned about the quality of the deal than we are about the clock, though we also know that difficult decisions won't get any easier with time - that is why we are continuing to negotiate," she added.
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.
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.
On Tuesday, European Union Foreign Policy Chief Federica Mogherini said the parties will continue negotiations for "the next couple of days" in an effort to hammer out a final nuclear deal.