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Last push

But British Foreign Secretary Philip Hammond said they would launch one more attempt to get a final agreement.

"At the moment we're focused on the last push, a big push tomorrow (Monday) morning to try and get this across the line," he told reporters. "Of course if we're not able to do it, we'll then look at where we go from there."

Some Western officials describe two possible options for a likely rollover. Under one scenario, described as the "stop the clock option", the talks would simply break off and experts from the parties would reconvene in a few weeks for another attempt.

A lengthier option would be a formal extension into next year, adding new elements to an interim accord from last year.

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International Crisis Group's senior Iran analyst Ali Vaez said there could be a "no-cost extension in which the parties would continue negotiating without discussing the terms of a new interim agreement or a firm deadline, with the hope of hammering out the final agreement by the year's end".

Several Western officials have questioned the value of repeatedly extending the talks, saying there is little reason to expect the Iranians will show the flexibility needed to end the impasse in the weeks and months ahead. They have questioned the Iranian leadership's desire to compromise.

Tehran blames the West for the deadlock, saying it has consistently made unreasonable demands of Iran.

They have also warned that the upcoming change in U.S. Congress, where hardline Republicans will soon dominate both houses, means U.S. lawmakers may push for new sanctions on Iran, which the Obama administration has said could torpedo the talks.

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Iran, powers set to extend nuclear talks