AFP/ Vienna

After months of intense negotiations, global powers and Iran launched into a key weekend of talks Saturday amid signs they may be within sight of a historic nuclear deal to end a 13-year standoff.

While all sides remained cautious, Iranian Foreign Minister Mohammad Javad Zarif said the world had "never been closer" to reaching an unprecedented accord with his country aimed at blocking Iran's pathways to developing nuclear arms.

In signs of a possible breakthrough on one of the thorniest issues still blocking a deal, the IAEA announced Saturday that it may be able to complete a probe into whether Iran has ever sought nuclear weapons by the end of the year.

In a rare move, Zarif also offered the promise of greater cooperation to tackle other global problems, such as the rise of the Islamic State group, should the deal be sealed.

Global powers are trying to draw the curtain on almost two years of roller-coaster negotiations, which gathered fresh impetus after President Hassan Rouhani took power in late 2013.

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, some of which date back to 1995.

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 the Islamic State (IS) jihadist group that has overrun parts of Syria and Iraq.

- Deal '90%' done -

Meanwhile, IAEA chief Yukiya Amano told reporters as he re-joined the nuclear negotiations that progress had been made as the International Atomic Energy Agency (IAEA) seeks to ensure that any deal is "technically sound."

"With the cooperation from Iran I think we can issue a report by the end of the year on the ... clarification of the issues related to possible military dimensions," Amano told reporters after returning from talks in Tehran earlier this week.

Iran has long denied accusations that it has sought to develop a nuclear bomb and has so far refused to give UN inspectors access to sensitive military sites to verify its claims.

The standoff has stalled an IAEA probe into the allegations that before 2003, and possibly since, Iran conducted research work into developing nuclear weapons.

Russian diplomats have said the complex accord, which will stretch to at least 20 pages with a slew of technical annexes, is "90%" written.

After missing several deadlines, the so-called P5+1 group -- Britain, China, France, Germany, Russia and the United States -- 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.

"We're certainly making progress, there's no doubt about that... but it's also clear there are still big issues not resolved, which is why people are burning the midnight oil."

Some outstanding points of contention have been concluded in the past few days. And with teams of experts working around the clock to nail down last hurdles -- including 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.