The deal, signed in November, saw Iran suspend parts of its contested nuclear programme for six months in return for limited sanctions relief while a longer term agreement is hammered out.
"If expert-level talks are fruitful, a date will be decided which I guess will be at the end of January" for the deal to be implemented by, said Abbas Araqchi, a deputy foreign minister, in comments run by Mehr.
The talks are aimed at setting a framework to implement the nuclear deal clinched in Geneva on November 24, which seeks to buy time to thrash out a diplomatic solution to a decade-long standoff over Iran's nuclear ambitions.
But Araqchi said the talks were "not that easy."
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"The negotiations are proceeding slowly as there are misunderstandings over interpretation of some elements of the accord," he said.
The first round of technical talks in Vienna were interrupted in early December when Iranians walked out after Washington expanded its existing sanctions blacklist against Tehran.
The second round was suspended as the Christmas holidays approached.
When the sides agree on how to implement an accord, Iran and world powers will open negotiations to hammer out a comprehensive deal to allay Western suspicions that Tehran's nuclear drive masks military objectives.