File photo: This picture shows John Kerry (L) and Mohammad Javad Zarif at a bilateral meeting of the closed-door nuclear talks. (AFP)
US Secretary of State John Kerry has warned it is too soon to tell if tough negotiations with Iran on a deal curtailing its nuclear programme will succeed.
On the eve of Tuesday's deadline, which everyone has already acknowledged will slip, Kerry met with Yukiya Amano, the head of the UN nuclear watchdog.
"We're just working and it's too early to make any judgements," Kerry told reporters when asked if the talks were making progress in Vienna.
One of the thorniest issues still blocking a deal is ensuring that the UN's International Atomic Energy Agency (IAEA) can finally report that Iran's nuclear programme is entirely peaceful.
Iran has long denied seeking to arm itself with a nuclear bomb, but is so far refusing to give the IAEA unparallelled access to sensitive military sites where nuclear research may have been happening.
Global powers - Britain, China, France, Germany, Russia and the United States - are seeking to end a 12-year standoff with Iran over its suspect nuclear programme.
After 20 months of intense talks, which have criss-crossed the globe, the parties have set a Tuesday deadline for a final accord, with several countries saying they are prepared to walk away if Iran cannot meet their demands.
Iranian Foreign Minister Mohammad Javad Zarif was back in Tehran after leaving Vienna late Sunday apparently to consult with the top Islamic leadership which will have the final say on any deal.
Behind-the-scenes meanwhile, work continues on drawing up what could be one of the world's most complicated nuclear non-proliferation agreements aimed at putting an atomic bomb out of Iran's reach.
The text is believed to be at least 20 pages long, accompanied by perhaps as many as five complicated technical annexes.
The deal is unlikely to be reached on Tuesday, with a US official admitting that everyone was planning to stay beyond the deadline day.
But EU foreign policy chief Federica Mogherini spoke for many late Sunday when she insisted there would be no formal months-long extension, saying "postponement is not an option".
"I would say that the political will is there. I've seen it from all sides," Mogherini said, adding "we have conditions now to close the deal" saying there were only a couple of point outstanding which needed political decisions.
Chinese Vice Foreign Minister Li Baodong told reporters in Vienna that a comprehensive deal could be possible within one week, saying all sides were only "steps away," the state news agency Xinhua reported.