Iran and six world powers looked set to miss Monday's deadline for resolving a 12-year stand-off over Tehran's nuclear ambitions and are already looking at a possible extension of the negotiations.
The talks in Vienna aim for a deal that could transform the Middle East, open the door to ending economic sanctions on Iran and start to bring a nation of 76 million people in from the cold after decades of hostility with the West.
The cost of failure to reach a deal could be high. Iran's regional foes Israel and Saudi Arabia are watching the Vienna talks nervously. Both fear a weak deal that fails to curtail Tehran's nuclear ambitions, while a collapse of the negotiations would encourage Iran to become a threshold nuclear weapon state, something Israel has said it would never allow.
It became increasingly clear during a week of intensive negotiations between the top US and Iranian diplomats that what officials close to the talks have been predicting privately for weeks is proving to be correct: a final deal is still too far off to hammer out by the parties' self-imposed deadline.
A European official said the possibility of securing a final agreement "seems physically impossible", echoing comments by Iranian officials.
With the deadline less than 24 hours away, the issue was one of several options for negotiations raised in US Secretary of State John Kerry's latest meeting with Iranian Foreign Minister
Mohammad Javad Zarif, a senior US official said on condition of anonymity.
The United States, Britain, France, Germany, Russia and China began the final round of talks with Iran on Tuesday to clinch a pact under which Tehran would curb its nuclear work in exchange for lifting economically crippling sanctions.
German Foreign Minister Frank-Walter Steinmeier spoke of a deep divide between Iran and the six powers, saying they were "still far apart on many issues".
