VIENNA - Iran, the United States and other world powers are all but certain to miss Monday's deadline for negotiations to resolve a 12-year stand-off over Tehran's atomic ambitions, forcing them to seek an extension, sources say.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 said: "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 said 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.TOO EARLY FOR FINAL JUDGMENTUS 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 meeting between Zarif, Kerry and Ashton was good," an unidentified Iranian official was quoted as saying by another Iranian news agency, IRNA. "However, we still have a lot to work on and it is too early for the final judgement."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 United States 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 United States and mainly Shi'ite Muslim Iran have found themselves on the same side on the battlefield against Sunni Muslim militants from Islamic State, especially in Iraq where both Washington and Tehran provide military support to the Baghdad government.But without a nuclear deal, two countries that have labelled 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 United States and ally Israel have said they reserve the right to use force to destroy any Iranian nuclear bomb program.Both US President Barack Obama, a center-left Democrat, and Iranian President Hassan Rouhani, a Shi'ite 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 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.Netanyahu said Israel was making clear its position to the world powers that Iran must not be allowed to be recognized as a nuclear threshold power. “There is not reason that it be allowed to keep thousands of centrifuges that will enable it to enrich uranium for a nuclear bomb in a short time,” he said.The prime minister told Sunday's weekly cabinet meeting that no nuclear deal with Iran is preferable to a bad deal that will endanger Israel, the Middle East and the entire world. He added that there was also no reason that Iran be allowed to develop intercontinental ballistic missiles that can carry a nuclear payload and threaten the entire world.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 time frame of lifting sanctions."