Talks between world powers and Iran on a historic nuclear deal have entered what France describes as the "final phase", but Washington warns that major issues must still be overcome.
Hopes grew that a breakthrough might finally be in sight after a flurry of diplomatic activity ahead of the latest deadline on Monday for an agreement.
"I hope we are finally entering the final phase of these marathon negotiations. I believe it," French Foreign Minister Laurent Fabius told reporters on Sunday as he returned to Vienna on the haggle's 16th day.
The talks seek to nail down a deal curbing Iran's nuclear activities to make it extremely difficult for Tehran - which denies any such goal - to develop the atomic bomb.
In return Iran will be granted staggered relief from painful sanctions, although the six powers insist on the option of reimposing the restrictions if Tehran breaches the deal.
Despite the air of optimism in the Austrian capital, US and Iranian officials dampened speculation that an agreement was imminent.
"We have never speculated about the timing of anything during these negotiations, and we're certainly not going to start now - especially given the fact that major issues remain to be resolved in these talks," a senior US State Department official said.
Iranian diplomat Alireza Miryousefi, writing on Twitter, quoted a senior official from Tehran as saying a deal by Sunday night was "logistically impossible" as the agreement being drawn up spanned 100 pages.
Earlier, US Secretary of State John Kerry, who has been embroiled in talks with his Iranian counterpart Mohammad Javad Zarif in Vienna since June 27, was cautiously upbeat.
"I think we're getting to some real decisions. So I will say, because we have a few tough things to do, I remain hopeful. Hopeful," Kerry said, calling his latest meeting with Zarif "positive".
EU foreign policy chief Federica Mogherini who chairs the P5+1 group - the US, Russia, China, Britain, France and Germany - negotiating with Iran said on Twitter that these were the "decisive hours".
And a diplomatic source said on Saturday that "98 per cent of the text is finished".
Under the parameters of a framework deal reached in Lausanne in April, Iran is to slash the number of its centrifuges from more than 19,000 to just over 6000 and sharply cut its stocks of enriched uranium.
Negotiators left the thorniest issues until last, including a mechanism for lifting interlocking EU, US and UN sanctions.
A new hurdle was thrown up in recent days, with the Iranian delegation insisting a UN arms embargo be lifted once a deal is reached.
The talks have also stumbled on demands to give UN nuclear inspectors access to military sites, to probe suspicions Iran sought to develop nuclear weapons in the past.