Iran, the United States and the European Union ended two days of high-level talks on Tehran's nuclear programme on Monday with no immediate sign they had bridged gaps ahead of a November 24 deadline for an agreement.
Last two days, US Secretary of State John Kerry and Iran's Foreign Minister Mohammad Javad Zarif sought to overcome deep differences, allay mutual suspicion and bring 12 months of diplomatic brinkmanship to the point of a breakthrough.
As each laid out their demands in private, warnings came that a final agreement may prove elusive, but in their only public comments in over 10 hours of talks neither man revealed what remains unsolved in the long-running bid for a comprehensive accord.
Asked if they were making progress, as they appeared briefly for photographers, Zarif replied: "We will eventually."
Kerry said: "We are working hard. We are working hard".
After the talks ended on Monday, the US State Department said they had proven "tough, direct and serious" while adding "there is still time" for progress.
Iran's deputy foreign minister Abbas Araghchi said "no progress" had been made during the two days.
"We can no longer talk about progress in the negotiations, but we are optimistic that we can reach an accord" before November 24, he said, quoted by ISNA news agency.
The Iranian delegation is under pressure to deliver a quick and total lifting of US, UN and European sanctions under a final deal. Obama, however, said sanctions would only be "slowly reduced" if Tehran meets its obligations.
The key sticking point is thought to be the number and type of uranium-enriching centrifuges Iran should be allowed to keep spinning in exchange for sanctions relief and rigorous inspections of its nuclear sites.
Iran denies it is seeking a bomb and says its nuclear programme aims to produce atomic energy to reduce the country's reliance on fossil fuels, requiring a massive increase in its ability to enrich uranium in coming years.
The duration of a final settlement plan between Iran and the P5 1 group - Britain, China, France, Russia and the United States plus Germany - also remains contested, with Iran speaking of five years and world powers suggesting at least double that.