Iran, the United States and other world powers are all but certain to miss Monday's deadline for negotiations to resolve a 12-year standoff over Tehran's atomic ambitions, probably forcing the parties involved to seek an extension, sources confirmed on Sunday.
Many believe the talks in Vienna could lead to a transformation of 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.
But sources confirmed on Sunday what officials close to the talks have been predicting privately for weeks: that a final deal is still too far off to hammer out by the deadline.
"Considering the short time left until the deadline and number of issues that needed to be discussed and resolved, it is impossible to reach a final and comprehensive deal by Nov. 24," Iran's ISNA news agency quoted an unidentified member of the country's negotiating team in Vienna as saying.
"The issue of extension of the talks is an option on the table and we will start discussing it if no deal is reached by Sunday night," the official said.
A European official who spoke to reporters on condition of anonymity told Reuters: "To reach a comprehensive deal seems physically impossible. Even if we were to get a political agreement the technical annexes are not ready."
A European source told Reuters on Saturday there was no decision yet on extending the talks: "It's the ministers' decision, but talks on an extension could begin Sunday or Monday," he said, adding that a rollover could run for several months.
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.
U.S. Secretary of State John Kerry met Iranian Foreign Minister Mohammad Javad Zarif and EU envoy Catherine Ashton on Sunday at a 19th-century palace in the center of Vienna, their fifth meeting since the talks began.
The talks aim to end Western suspicions that Iran is seeking an atomic bomb capability, while allowing Iran to have the civilian nuclear program it says is its right under international treaties.
In a breakthrough preliminary deal reached a year ago, the U.S. and European Union agreed to ease some sanctions on Iran while Tehran agreed to some curbs on its nuclear programs. But a final deal proved elusive, with the sides forced to extend an earlier deadline in July.
Last year's negotiations opened secret talks between Tehran and Washington, which have transformed relations between two countries whose deep enmity has been one of the central facts of the Middle East since Iran's 1979 Islamic revolution.
This year, the U.S. and mainly Shia Muslim Iran have found themselves on the same side on the battlefield against Sunni Muslim militants from Islamic State of Iraq and the Levant (ISIL), especially in Iraq where both Washington and Tehran provide military support to the Baghdad government.
But without a nuclear deal, two countries that have labeled each other the "Great Satan" and a member of the "axis of evil" are destined to remain enemies.
Sanctions, tightened sharply since 2010, are inflicting severe damage to Iran's economy, while the U.S. and ally Israel have said they reserve the right to use force to destroy any Iranian nuclear bomb program.
Both President Barack Obama and Iranian President Hassan Rouhani, a Shia cleric elected on a pledge to reduce Iran's isolation and improve the economy, would have to sell any deal to skeptical hardliners at home.
Washington would also have to win acceptance from regional allies Israel and Saudi Arabia, both foes of Iran. Kerry briefed Israeli Prime Minister Benjamin Netanyahu by phone on Saturday and Saudi Arabia's Foreign Minister Saud al-Faisal in person at the Vienna airport on Sunday.
"Iran must not be allowed to set itself up as a nuclear threshold state," Netanyahu said about his conversation. "There is no reason for it to retain thousands of centrifuges which would allow it to enrich uranium for a nuclear bomb in a short period of time."
Officials say an extension of the talks could last from several weeks to several months, depending on how close a deal seems by the end of Monday. Neither side wants the negotiations to collapse, but Western officials say they are afraid extending the talks again could make it even harder to get a final deal.
Iranian and Western diplomats close to the negotiations in Vienna have been telling Reuters for weeks that the two sides remained deadlocked on the key issues of Iran's uranium enrichment capacity and the pace of lifting sanctions.
The Iranian official quoted by ISNA said the sides "were trying to reach a framework accord on major issues like ... the number of centrifuges, enrichment capacity and the timeframe of lifting sanctions."