VIENNA - Foreign ministers from major powers were set Thursday to turn the screws on Iran to finalise a historic nuclear deal on the eve of a deadline to present it to US lawmakers.
If the US Congress does not receive the text by early Friday morning Vienna time - midnight in Washington - it makes the approval process longer and potentially more problematic.
But despite this pressure, and almost two weeks of talks in the Austrian capital, it is unclear whether an accord aimed at ending a 13-year standoff can be sewn up at all, let alone in time.
Building on a framework agreement from April, the deal would see Iran dismantle large parts of its nuclear infrastructure in order to put a nuclear bomb out of reach.
In return, a web of painful United Nations and Western sanctions on Iran - which denies wanting the bomb - would be progressively lifted once the UN atomic watchdog verified Tehran had fulfilled its promises.
"We have come a long way over the past 21 months of negotiations over my country's nuclear energy programme," Iranian Foreign Minister Mohammad Javad Zarif wrote in the Financial Times.
"Never have Iran and its counterparts been this close to a final accord. But success is far from assured... Serious political decisions still need to be made," he said.
Difficult issues in what will be a highly complex agreement include working out the pace and timing of sanctions relief and a stalled UN probe into alleged efforts by Iran to develop the atomic bomb.
Iran has also insisted there should be changes to a UN arms embargo and an easing of restrictions on missile sales, a prospect alarming rivals of Iran and allies of the United States in the region.
'Never threaten an Iranian'
In a sign of tensions, diplomats in Vienna told of a stormy meeting Monday evening between US Secretary of State John Kerry, Zarif and the foreign ministers of the other major powers.
Iranian media reported that Zarif told EU foreign policy chief Federica Mogherini: "Never threaten an Iranian". Diplomats said there was shouting during a one-on-one meeting between Kerry and Zarif.
The foreign ministers of Britain, France and Germany were expected to rejoin the talks on Thursday morning. It was unclear when their Russian and Chinese counterparts might return.
A deal with the P5+1 group - the United States, Russia, China, Britain, France and Germany - could pave the way to normalised relations between Iran and the West after 35 years of mistrust and enmity.
It might also lead to increased cooperation in the Middle East, for example in fighting Islamic State militants - a common enemy of Washington and Tehran - who have overrun large swathes of Iraq and Syria.
If Congress does not get the deal in time, the review period doubles to 60 days, giving opponents more time to reject it. During this time US President Barack Obama cannot waive Congressional sanctions, the most painful ones for Iran.
If they do reject it, then Obama has 12 days to accept or veto. Congress then has another 10 days to override the veto, meaning the entire process could take up to 82 days.
Back in Iran, Iranians are anxiously following the news out of Vienna, hoping for a deal that will end the sanctions that have crippled the country's economy.
"It's important to get it done as soon as possible, because the longer it takes, the more money and opportunities we lose" to boost the economy, said Mohammad, a 31-year-old computer engineer from the northeastern city of Shahrud.