Talks on Iran's nuclear programme appear to have moved up a gear with US Secretary of State John Kerry and Iran's foreign minister agreeing to stay on in Vienna to try and break a deadlock just days before an agreed deadline.
Kerry had been set to travel to Paris earlier on Friday, while sources said Mohammed Javad Zarif had been considering returning to Tehran, but both remained in the Austrian capital.
"Earlier this afternoon, it looked as though the talks had reached a point beyond which they can go no further," Al Jazeera's Jonah Hull, reporting from the Austrian capital, said.
"But it seems that there is a bit of momentum back in these talks."
Jen Psaki, Kerry's spokesperson, said the secreatary of state delayed his departure "in order to continue consultations" with Zarif.
"We have not yet determined when we will depart, but will stay in Vienna overnight," she said on Friday before the two men went into a meeting lasting for more than two hours.
Zarif, whose possible trip back to Iran for "consultations" had suggested possible progress, said that world powers had made "no significant propositions worth returning to Tehran with".
Iran and six powers - the US, China, Russia, Britain, France plus Germany (P5+1) - have been negotiating intensively since February to turn an interim accord reached with Iran a year ago into a lasting agreement by November 24.
Such a deal, after 12 years of rising tensions, is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities - an ambition Iran denies.
Fears had been growing that the Monday deadline for a full deal would be missed because of differences on how much Iran needs to reduce the size and scope of key parts of its nuclear programme.
British Foreign Secretary Philip Hammond left Vienna in the afternoon, saying there was still a "very significant gap".
He earlier said that he was "not optimistic" the deadline could be met, suggesting that the best hope was for another extension.
But Kerry had insisted on Thursday this was not on the table.
"We are not discussing an extension. We are negotiating to have an agreement. It's that simple," Kerry said in Paris before he went to Vienna.
Iran believes that the onus is on the side of the world powers to compromise.
Kelsey Davenport, an expert at the Arms Control Association, said there was "still a lot of time until Monday at midnight".
She told Al Jazeera she believed it was a "very positive sign" that the US delegation and the Iranians decided to stay in Vienna.
Source: Al Jazeera and agencies