Iran and world powers were set for new talks Thursday on how to implement a landmark deal aimed at containing Tehran's nuclear drive, just days before the agreement is due to take effect.

Russian President Vladimir Putin and his Iranian counterpart Hassan Rouhani discussed the November accord in a phone conversation Thursday shortly before the opening of the negotiations in Geneva.

Iranian Foreign Minister Mohammad Javad Zarif has voiced optimism about the talks but some observers have warned of possible sticking points that could lead to a delay in rolling out the deal.

The Kremlin said Putin and Rouhani discussed a range of issues, including "the implementation of agreements on the Iranian nuclear programme".

Iranian, EU and US negotiators are gathering in Geneva for their highest-level talks since hammering out the groundbreaking November 24 deal aimed at reining in Tehran's nuclear ambitions.

Negotiators have said they aim to implement the deal -- which also offers Iran some sanctions relief -- by January 20.

Little information has filtered out about the two-day talks, but they were expected to focus heavily on the thorny issue of advanced centrifuges.

The European Union, which represents the so-called P5+1 group of world powers -- Britain, China, France, Russia and the United States plus Germany -- has not even divulged where in Geneva the meeting is taking place.

Iran's deputy chief nuclear negotiator Abbas Araqchi and Helga Schmid, deputy to EU foreign policy chief Catherine Ashton, will "discuss outstanding issues" on implementing the deal was all Ashton's spokesman Michael Mann would tell AFP.

Top US nuclear negotiator Wendy Sherman will meet with both Araqchi and Schmid, the State Department said, without confirming reports there would be a three-way encounter.

Western powers and Israel fear Iran is seeking to develop the atomic bomb under the guise of a civilian nuclear programme but this has always been denied by Tehran.

In the November deal, Iran agreed to curb parts of its nuclear drive for six months in exchange for receiving modest relief from international sanctions and a promise by Western powers not to impose new measures against its hard-hit economy.

Technical experts from Iran and the P5+1 have since held several sessions in Geneva aimed at finetuning the deal.

Zarif, also Iran's top nuclear negotiator, voiced optimism, saying on Facebook that "the nuclear talks are continuing with seriousness and a strong political will".

But other observers hinted the issue of a new model of nuclear centrifuges, which could potentially enable Iran to rapidly purify uranium to a weapons-degree level, might become a sticking point.

'Open to interpretation'

A Vienna-based envoy told AFP the issue of advanced centrifuges that Iran is conducting research on was "one of the items still to be decided" and "is being debated a lot".

Two weeks ago, Iran's atomic energy chief Ali Akbar Salehi said Tehran was "testing third and fourth generations of its centrifuges," which were almost five times more effective that the current ones.

The problem, according to the Vienna-based diplomat, was that the November plan "wasn't that specific" on the issue, meaning it is "open to interpretation by both sides".

"It's a major issue but whether it will become a major sticking point, we will only see over the next few days," the diplomat said, voicing optimism that "we will find some common ground."

Siavush Randjbar-Daemi, a Middle East and Iran lecturer at Manchester University, also said the different interpretations of the Geneva accord, and Iran's stated willingness to install the new centrifuges, was an issue.

"I am... of the view that the West feels that Iran's attitude needs to shift," he told AFP in an email.

Tehran too has signalled it was not happy with its counterparts since the November breakthrough.

When experts held four days of talks last month in Vienna -- home of the International Atomic Energy Agency -- the Iranians walked out after Washington expanded its sanctions blacklist against Tehran.

The interim deal is meant to buy time for diplomacy to clinch a lasting agreement that would allay Western suspicions that Iran is covertly developing nuclear weapons.

And neither Israel -- widely considered to be the Middle East's sole if undeclared nuclear-armed state -- nor Washington have ruled out military action.

The talks, which hit a wall amid tensions between the West and Iran's hardline former president Mahmoud Ahmadinejad, gathered pace after the election of relative moderate Hassan Rouhani, who succeeded him last August.