Iran's six-month temporary rollback of its nuclear activities agreed to in the recent deal has not begun and the start date has not yet been set, a spokesman for world powers' chief negotiator said Wednesday.

"The actual date for the beginning of the six-month period of the first step has yet to be decided," Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, told AFP.

"It will also depend on the outcome of technical discussions with Iran about the implementation arrangements that will take place soon," he said.

In a major breakthrough Sunday in Geneva, Iran agreed with the United States, China, Russia, Britain, France and Germany -- the P5+1 -- to freeze some its nuclear work in exchange for minor sanctions relief.

This six-month stop is meant to make it more difficult for Iran to develop a nuclear weapon and to build confidence while Tehran and the P5+1 hammer out a long-term accord.

One Western diplomat in Vienna, headquarters of the UN atomic watchdog, said they expected the six months to begin in January and that the expert-level talks would "likely" take place next week.

"It is all understandably vague but my understanding is that the six months start in January," the diplomat said on condition of anonymity.

"The experts are going to be talking next week about how you translate that document (Sunday's deal) into something which is more specific and practical.... The momentum behind this is really very strong. Everyone realises we have a limited opportunity to get this right."

The six powers, Iran and UN watchdog the International Atomic Energy Agency are to create a Joint Commission to work out exact timing and how the IAEA will monitor Iran's compliance.

A second diplomat told AFP that these technical and logistical details "need to be worked out in the coming days" but that a "specific timeframe for these meetings is still being determined".

The freeze involves Iran limiting uranium enrichment to low fissile purities for half a year and lowering the purity or converting to another form its entire stockpile of medium-enriched material, which is relatively easy to convert to weapons-grade.

Iran also pledged for six months "not to make further advances" at its Fordo and Natanz enrichment sites and its Arak reactor and to submit to what US Secretary of State John Kerry called "unprecedented" IAEA inspections.