By John Irish and Fredrik Dahl
VIENNA, Nov 22 (Reuters) - World powers and Iran struggled on Saturday to overcome the differences preventing them from ending a 12-year standoff over Tehran's atomic ambitions, raising the prospect of another extension to the high-stakes talks.
U.S. Secretary of State John Kerry said "big gaps" remained with two days to go before a self-imposed Nov. 24 deadline for an accord, despite signs of some headway. A European source said the chance of a final deal by Monday was "very small."
The negotiations in Vienna between Tehran and six world powers - the United States, Britain, France, Germany, Russia and China - are intended to remove at least one source of potential conflict from the Middle East and its growing turmoil.
Diplomats said a framework accord was still possible, but that weeks or months would then be needed to agree on the all-important details of how it would be implemented.
They said continuing the negotiations - which have dragged on for more than a year, accompanied by a partial easing of sanctions - was preferable to letting them collapse and risking renewed tension. However, some said an extension could push the talks into a never-ending cycle of rollovers with few prospects of a final deal.
Despite the differences, a senior U.S. State Department official said: "We are continuing to take steps forward."
"The focus of discussions remains on an agreement, but we are discussing both internally and with our partners a range of options for the best path forward," the official added, without elaborating on whether these included a possible extension.
The European source said there had been "no significant" progress on the main stumbling blocks - Iran's uranium enrichment capacity and the lifting of the sanctions imposed on Iran over its nuclear program.
"The chances of reaching a deal in the next 48 hours are very small," the source said. "Our feeling is that they (Iran's negotiators) don't have a lot of flexibility."
On Friday, diplomatic sources had said Kerry and Iranian Foreign Minister Mohammad Javad Zarif were discussing new ideas to unblock the negotiations.
KERRY BRIEFS ALLIES
Kerry, Zarif and European Union envoy Catherine Ashton met again on Saturday.
"We hope we're making careful progress," Kerry said before a meeting with German Foreign Minister Frank-Walter Steinmeier. "But we have big gaps. We still have some serious gaps, which we're working to close."
Diplomats said the foreign ministers of France, Russia and China might join the talks on Sunday for what could be a final push to clinch some sort of agreement.
Iran rejects Western allegations that it has sought to develop an atom bomb capability, something that Iran's enemy Israel regards as an existential threat, and says the program is purely peaceful.
Western officials say Iran is refusing to countenance curbing uranium enrichment, an activity that can have both civilian and military uses.
Another stumbling block is sanctions, which Iran wants ended swiftly and not, as the West wants, suspended and then scrapped progressively as Iran fulfills the terms of a final deal.
Earlier on Saturday, Kerry held a conference call with the foreign ministers of the United Arab Emirates, Kuwait, Qatar and Bahrain, and separate calls with the foreign ministers of Turkey and Canada, a senior U.S. State Department official said.
He also spoke with Israeli Prime Minister Benjamin Netanyahu, who has repeatedly voiced his skepticism about any rapprochement with Iran. Tehran says it is Israel's unacknowledged nuclear arsenal that threatens regional peace and stability. (Additional reporting by Louis Charbonneau, Parisa Hafezi and Jonathan Allen; Editing by Raissa Kasolowsky)