VIENNA - US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif are discussing new ideas aimed at breaking the deadlock in nuclear talks between Tehran and six world powers, sources close to the talks said on Friday.The sources, who spoke to Reuters on condition of anonymity, said the top Iranian and US diplomats were preparing ideas that could be shown to both countries' capitals."Discussions are continuing," said a senior US official, who declined to discuss details about the negotiations in Vienna.One source said it was too early to say whether this latest joint effort indicated an actual narrowing of the wide differences between Iran and the six powers.The United States, Britain, France, Germany, Russia and China began a final round of talks with Iran on Tuesday, looking to clinch a pact under which Tehran would curb its nuclear work in exchange for a lifting of economically crippling sanctions. Iran rejects Western allegations that it has been seeking to develop a nuclear bomb capability.Officials close to the negotiations in Vienna said at mid-week the two sides remained deadlocked on key issues, were unlikely to secure a definitive accord by November 24, and might need to extend the deadline.Separately, sources close to the talks said Oman's Foreign Minister Yusuf bin Alawi was in Vienna, though Western officials said he was not playing an active mediation role in relation to the latest ideas under discussion by Kerry and Zarif. Oman was a key intermediary when Tehran and Washington launched secret talks on a possible nuclear deal more than a year ago.British Foreign Secretary Philip Hammond said earlier on Friday in Vienna: "These are complex issues and there are still significant gaps between the parties. We're all going away to have technical discussions with our experts and we'll resume again over the ... weekend."'ADDITIONAL EFFORTS'A senior Western diplomat said he was not optimistic about prospects for a long-term agreement by Monday's deadline.Russian Foreign Minister Sergei Lavrov and Kerry agreed that "additional efforts" were needed to reach a deal by the self-imposed deadline, Russia's Foreign Ministry said."The sides did not rule out the possibility of holding a ministerial meeting of the parties to the talks on Iran's nuclear program, if the prospect for progress appears," the ministry said in a statement.A senior Western diplomat declined to comment on the nature of the discussions between Zarif and Kerry, but said "we are still very far away" from an agreement.Western powers suspect Iran has aimed to covertly acquire a nuclear bomb capability from its enrichment of uranium. Iran says the prograe is for producing civilian energy only. The lingering dispute has raised fears of a wider Middle East war.Zarif cancelled announced plans to return to Tehran for top-level discussions with the deadline looming, Iranian media reported. The reason was not immediately known."The talks have not reached a stage that necessitates Zarif to go to Tehran," an unidentified senior member of the Iranian delegation told the IRNA and ISNA news agencies.US and French officials said earlier that Kerry and French Foreign Minister Laurent Fabius would return to Paris later in the day for consultations. But they later said both ministers would stay overnight in the Austrian capital.A source close to the talks said that Zarif had received a document from the powers that outlined the main principles of a possible agreement.