The talks, first announced in a report from Iran on Thursday, will be at the level of senior policy officials rather than foreign ministers, the European Union's diplomatic service said.
"The political directors of the (world powers) and Iran will meet again on 17 December 2014 in Geneva for a one-day meeting to continue diplomatic efforts towards reaching a long-term, comprehensive solution," it said in a statement.
Washington confirmed that acting Deputy Secretary fo State Wendy Sherman would first hold bilateral talks with the Iranian delegation in the Swiss city on Monday and Tuesday.
The US-Iran bilateral talks would be joined for part of the meeting by the EU's Helga Schmid, it added.
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It will be the first time the sides have met since failing to meet a November 24 deadline for a full deal with Iran on reining in its nuclear programme in exchange for an easing of crippling international sanctions.
The EU's external affairs arm said the P5+1 talks in Geneva next week would be technical, not political.
Accordingly, Ashton, who chaired the talks previously and now serves as special Iran adviser to her successor Federica Mogherini, will not attend the talks, a spokeswoman said.
Mogherini announced today that Ashton would continue to lead the negotiations with Iran so as to ensure continuity.
A final agreement is aimed at ensuring Tehran will never develop nuclear weapons under the guise of its civilian activities.