By John Irish and Fredrik Dahl
VIENNA (Reuters) - World powers and Iran struggled on Saturday to overcome crucial differences that are preventing them from ending a 12-year standoff over Tehran's atomic ambitions, raising the prospect of another extension to the high-stake 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 likelihood of a final deal by Monday was "very small".
Diplomats said a framework accord was still possible, but that weeks if not months would then be needed to agree on the all-important details of how it would be implemented.
They made clear that continuing the negotiations - which have dragged on for more than a year - was preferable to letting them collapse and risking renewed tension. However, diplomats warned that an extension could push the talks into a never-ending cycle of rollovers with few prospects of a final deal.
The negotiations in Vienna are intended to resolve a long-running dispute between Iran and the West and remove at least one source of potential conflict from the Middle East and its growing turmoil.
"The chances of reaching a deal in the next 48 hours are very small," the European source said. "Our feeling is that they (Iran's negotiators) don't have a lot of flexibility."
There had been "no significant" progress on the main stumbling blocks of Iran's uranium enrichment capacity and the lifting of the sanctions imposed on Iran over its nuclear program, the source said.
Diplomatic sources said on Friday that Kerry and Iranian Foreign Minister Mohammad Javad Zarif were discussing new ideas to unblock the negotiations between Tehran and six powers: the United States, Britain, France, Germany, Russia and China.
Kerry, Zarif and European Union envoy Catherine Ashton met again on Saturday.
Officials had said earlier in the week that deadlock remained on key issues, and that the deadline, already extended by four months along with a partial easing of sanctions, might need to be pushed back again.
"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."
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.
KERRY BRIEFS GULF ALLIES, ISRAEL
Western officials say Iran is not budging on key issues such as uranium enrichment, an activity that can have both civilian and military uses.
They say Iran has refused to reduce its enrichment capacity, which Western officials say would leave it with the capacity to amass enough material for an atomic bomb in a few months. Washington wants this "breakout" timeline extended to at least a year.
Another stumbling block is sanctions, which Iran wants ended swiftly and not, as the West wants, suspended and scrapped progressively as Iran fulfils the terms of a final deal.
Iran also objects to Western demands that such a deal should last up to 20 years.
But diplomats say the six powers are likely to relent on demands for full disclosure of any secret weapon work by Tehran, in the interest of securing a 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 scepticism about any rapprochement with arch foe Iran. Tehran says it is Israel's atomic arsenal that threatens regional peace and stability.
(Additional reporting by Louis Charbonneau, Parisa Hafezi and Jonathan Allen; Editing by Raissa Kasolowsky)