The United States, Britain, France, Germany, Russia and China began the final round of talks with Iran on Tuesday, looking to clinch a pact under which Tehran would curb its nuclear work to help ensure it cannot be diverted to bomb-making in exchange for a lifting of economically crippling sanctions.

But 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 Nov. 24, and might need to extend the deadline.

Russian Foreign Minister Sergei Lavrov and Kerry agreed "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.

British Foreign Secretary Philip Hammond said after separate meetings on Friday with big power peers 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."

Western powers suspect Iran has aimed to covertly acquire a nuclear bomb capability from its enrichment of uranium. Iran says the program is for producing civilian energy only. The decade-long dispute has raised fears of a wider Middle East war.

Iranian Foreign Minister Mohammad Javad Zarif canceled announced plans to return to Tehran for top-level discussions with the deadline looming, Iranian media reported. The reason for his reversal of course was not immediately known.

"The talks have not reached a stage that necessitates Zarif to go to Tehran," an unnamed senior member of the Iranian delegation told the IRNA and ISNA news agencies. "Therefore he is not going to Tehran and the talks will continue."

U.S. and French officials said earlier that U.S. Secretary of State John Kerry and French Foreign Minister Laurent Fabius would return to Paris later in the day for consultations.

Kerry spoke on the telephone with Russian Foreign Minister Sergei Lavrov on Friday and the two men agreed that "additional efforts" were needed to secure a deal by Monday, Moscow's Foreign Ministry said.

A source close to the talks told Reuters that Zarif had received a document from the powers that outlining the main principles of a possible agreement on removing sanctions on Tehran in exchange for limits on its nuclear program.

STICKING POINTS

Western diplomats told Reuters earlier this week that a U.S.-drafted proposal shown to Iran at preparatory talks in Oman earlier this month called for the Islamic Republic to reduce the number of its uranium enrichment centrifuges to 4,500, well below the current 19,000 Tehran now has in operation.

Iran has about 10,000 of those machines in operation. Iranian officials have refused to reduce the volume of uranium they are capable of enriching, a stand Western officials say is unacceptable as this would potentially allow Tehran to amass enough fissile material for an atomic bomb in little time.

This is a major sticking point in the talks.

Another sticking point is the pace and sequencing of sanctions relief. Iran wants them terminated swiftly, not suspended and gradually scrapped, depending on the degree of Iranian compliance with the deal terms, as the West wants.

Another dispute is over the deal's duration - the powers want it to be up to 20 years, Iran wants this much shorter.

It was not clear when Kerry would be back in Vienna. State Department spokesman Jen Psaki said he was leaving "for consultations with his European counterparts about the ongoing Iranian nuclear negotiations".

Aides said Fabius would return to Vienna over the weekend.

"We have to get more flexibility from the Iranians," Britain's Hammond told reporters upon arrival. "In return we are prepared to show some flexibility on our side. But time is short, we are up against a deadline here."

An interim accord struck on Nov. 24 last year yielded steps by Iran to curb some sensitive nuclear activity, such as higher-grade enrichment, in return for some sanctions relief. That pact will expire on Monday.

(Additional reporting by Parisa Hafezi, Louis Charbonneau and Fredrik Dahl; Editing by Mark Heinrich)