Al-Mayadeen news channel quoted a diplomat as saying that "certain difficulties have made it difficult to declare a final deal tonight".
In relevant remarks on Monday, Iranian Deputy Foreign Minister and senior negotiator Seyed Abbas Araqchi said he doesn't know if the remaining differences between Tehran and the world powers would be resolved or not.
Iran and the Group 5+1 (the US, Russia, China, Britain and France plus Germany) are in the final phase of their last round of talks in Vienna to draft a final deal to end their over-a-decade-long nuclear standoff.
Al-Alam correspondent in Vienna reported that :Timetable for lifting restrictions on Iran nuclear program is one of obstacles in the negotiations.
Meanwhile Flynt Leverett, a senior US political analyst, believes that all parties in talks between Iran and the world powers are waiting for the White House to show flexibility in the nuclear talks to reach a final agreement.
"If the White House gives the American negotiators the flexibility to make the changes in the US positions that are needed to get to an agreement, I think it would be possible to finish over the next couple of days.
But I honestly don’t know if the White House is ready to do that," Leverett, a former senior fellow at the New America Foundation in Washington, D.C. and a professor at the Pennsylvania State University School of International Affairs, told FNA on Monday.