Such an extension would be under the terms of the Geneva accord that traded a temporary freeze on some aspects of Iran's nuclear activities for limited sanctions relief, the source told AFP.
"We are still focused on agreeing to a kind of political agreement" which would not be written but which would allow for negotiators to fine-tune technical aspects of the agreement later, the source said amid tense marathon talks in Vienna.
"But if between now and this afternoon or this evening we don't get there, the solution is we consider an extension of the Geneva accord," he said.
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"That could be for a period of six months or a year. We must absolutely avoid a climate of confrontation with escalation from one side and the other," the source said.
The United States, Russia, China, Britain, France and Germany have been locked in talks with Iran since February to turn the interim Geneva accord reached a year ago into a lasting agreement by November 24.
Such a deal is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities.
The Islamic Republic hotly denies such an aim and insists its programme is entirely peaceful.

