Nuclear talks between Iran and six world powers have moved up a gear as negotiators in Vienna began to hammer out what could be a historic accord.
After three meetings this year that Washington says have enabled both sides to "understand each other's positions", negotiators aim this time to start drafting the actual text of an accord, officials said.
Success could resolve one of the most intractable geopolitical problems of the 21st century, but failure might plunge the Middle East into conflict and start a regional nuclear arms race.
"If the odds of the talks collapsing are high, the stakes of failure are higher," Ali Vaez, Iran analyst at the International Crisis Group, told AFP.
"Time is of the essence."
The five permanent members of the UN Security Council plus Germany want Iran to take steps to assure the international community that it is not about to build a nuclear bomb.
In return the Islamic republic, which says its nuclear activities are purely peaceful, wants the lifting of all UN and Western sanctions, which have hit its economy hard.
Iranian Foreign Minister Mohammad Javad Zarif said after the last round in early April that there was agreement on "50-60 per cent" of issues.
But with both sides sticking to the mantra that "nothing is agreed until everything is agreed" - US officials liken the process to a "Rubik's Cube" - this is not enough.
Arriving in Vienna on Tuesday, representatives of both Iran and the United States sought to dampen expectations that a deal was within easy reach, with Zarif saying a "lot of effort" was still required.
A senior US official said the talks would be "very, very difficult" and that there were still "significant gaps", warning that optimism in some media had "gotten way out of control".
