Iran's Foreign Minister Mohammad Javad Zarif has said that the nuclear talks may be extended beyond its July 20 deadline after meeting with top diplomats from six world powers.
Zarif said on Tuesday that "serious differences" remained in nuclear talks with the United States, Britain, France, Germany, Russia and China.
But he said that the text of a draft accord on curbing Iran's nuclear programme in exchange for lifting sanctions had been improved significantly recently.
There is "an inclination among (the six powers) that more time may be useful", Zarif told reporters after three days of meetings with US Secretary of State John Kerry.
He added, however, that a decision on prolonging the negotiations had not been taken. There was no immediate comment from the six powers.
Tuesday's talks in the Austrian capital, Vienna, aimed to strike a deal meant to put firm curbs on Tehran's nuclear activities in exchange for an end to sanctions and reaching a long-term agreement.
Kerry said the negotiations would continue until at least Sunday. In the meantime, he said, he would consult with President Barack Obama and the US Congress about the state of the negotiations and the possibility of extending talks past the deadline so negotiations could continue.
"There has been tangible progress on key issues," Kerry told reporters. "However there are very real gaps on other key issues."
"While there is a path forward, Iran needs to choose to take it," said Kerry, adding he believed the two sides can determine "the precise contours of that path".
The main dispute is over Iran's nuclear enrichment programme.
Tehran says it needs to expand enrichment to make reactor fuel but the US fears Tehran could steer the activity toward manufacturing the core of nuclear missiles and could use its present capacity to produce enough weapons-grade uranium, Reuters news agency reported.