Russian Deputy Foreign Minister Sergei Ryabkov said on Thursday that he could not rule out the possibility of an agreement in the coming hours, even if there was still doubts over some of the finer details of the deal and if it could be resolved by the end of the day.
Iran's Deputy Foreign Minister Abbas Araqchi said the main text of the agreement, as well as five technical annexes, were "around 96 percent complete."
While the lifting of sanctions was largely agreed, Araqchi said Tehran's demand for an end to a UN Security Council arms embargo was among the most contentious unresolved points.
But an Iranian state broadcaster Press TV cited an Iranian official as saying it was unlikely an agreement would be reached on Thursday. A senior Western diplomat similarly also said it was "very doubtful" the talks would finish on Thursday.
Over the past two weeks, Iran, the United States, Britain, France, Germany, Russia and China have twice extended a deadline for completing a long-term deal under which Tehran would curb sensitive nuclear activities for more than a decade in exchange for sanctions relief.
Western countries accuse Iran of seeking the capability to build nuclear weapons, while Tehran says its programme is peaceful. A deal would depend on Iran accepting curbs on its nuclear programme in return for the easing of economic sanctions imposed by the United Nations, United States and European Union.
A successful deal could be the biggest milestone in decades towards easing hostility between Iran and the United States.
It would also be a political success for both US President Barack Obama and Iran's President Hassan Rouhani.
The White House said President Barack Obama and his national security team held a video conference on Wednesday with Kerry, Ernest Moniz and the US negotiating team in Vienna.
Tehran says the UN embargo on conventional weapons must be lifted in a nuclear deal. Western countries are keen not to allow Iran to begin importing arms because of its role supporting sides in conflicts in the Middle East.
Iran has powerful support on this issue from Russia. Foreign Minister Sergei Lavrov said the UN arms embargo should be among the first sanctions lifted in a deal.
"We are calling for lifting the embargo as soon as possible and we will support the choices that Iran's negotiators make," he said at a summit of BRICS countries - Brazil, China, India, Russia and South Africa.
