Global powers and Iran have started key talks in Vienna amid signs they may be inching towards a historic nuclear deal to end a 13-year standoff.
Iranian Foreign Minister Mohammad Javad Zarif on Friday warned there was still "no guarantee" of success, though he said the world had "never been closer" to reaching an accord with his country.
As Zarif met US Secretary of State John Kerry for fresh talks on Friday, the top US diplomat praised "the genuine effort" being made by all sides and insisted progress had been made despite "tough issues".
In a rare move, Zarif offered the promise of greater cooperation to tackle other global problems, such as the rise of the Islamic State of Iraq and the Levant (ISIL), should the deal be sealed.
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The aim is to finalise a deal which would put a nuclear bomb beyond Iran's reach, in return for lifting a web of biting international sanctions slapped on the Islamic republic.
Speaking in English from the balcony of the Viennese hotel hosting the talks, Zarif said in a message posted on YouTube that at "this 11th hour, despite some differences that remain, we have never been closer to a lasting outcome".
"Getting to yes requires the courage to compromise, the self-confidence to be flexible, the maturity to be reasonable," he said.
But he added that Iran was ready to strike "a balanced and good deal" which could "open new horizons to address important common challenges".
"Our common threat today is the growing menace of violent extremism and outright barbarism," he said in a clear reference to ISIL.
'In the endgame'
After missing several deadlines, the so-called P5+1 group - US, UK, China, France, Russia, and Germany - have now given themselves until Tuesday to conclude the deal.
"We're really in the endgame of all this," a senior US administration official told reporters.
With teams of experts working around the clock to resolve some of the thorniest issues - such as the extent and timing of sanctions relief, and how to win access to suspect Iranian nuclear sites - many of the ministers are due back in Vienna on Sunday.
It will be up to the UN watchdog, the International Atomic Energy Agency (IAEA), to verify Iran is sticking to its side of the bargain through enhanced inspections of its nuclear facilities.
But the P5+1 also want the IAEA to be able to visit sites where there is no declared nuclear material to probe alleged efforts, before 2003 and possibly since, to develop a nuclear weapon in secret.
On Thursday the IAEA chief Yukiya Amano visited Tehran to meet Rouhani, hoping to jumpstart a stalled probe into the so-called "possible military dimensions" of Iran's activities.
But after he returned, an IAEA statement suggested there had been no breakthrough.