Iran and world powers met Thursday to discuss how to implement a landmark deal aimed at containing Tehran's nuclear drive, less than two weeks before the agreement is due to take effect.
Iranian, EU and US negotiators gathered in Geneva for their highest-level talks since hammering out the groundbreaking November 24 deal.
Russian President Vladimir Putin and his Iranian counterpart Hassan Rouhani also discussed the implementation of the accord in a phone conversation earlier in the day, according to the Kremlin.
Rouhani warned, without elaboration, against "seeking excuses that would create problems in the negotiation process," Iran's ISNA news agency reported.
He also called on "certain countries … to respect their own commitments (under the Geneva deal) and avoid new strictures that would shadow their goodwill."
Negotiators have said they want to implement the deal, which aims to rein in Tehran's nuclear programme in exchange for some sanctions relief, by January 20.
Iranian Foreign Minister Mohammad Javad Zarif voiced optimism ahead of the talks, but some observers warned of possible sticking points that could lead to a delay in rolling out the deal.
Little information has filtered out about the Geneva talks, which were scheduled to continue Friday, 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 kept a tight lid on details about when and where the discussions were 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.
The US State Department said top US nuclear negotiator Wendy Sherman had met with both Araqchi and Schmid to provide "views and information that was useful to discussions to address any remaining issues to the agreement to the joint plan of action."
Iran and the US also held a bilateral meeting.
Western powers and Israel fear Iran is seeking to develop the atomic bomb under the guise of a civilian nuclear programme but Tehran has always denied this.
Under 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.
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Technical experts from both sides have since held several sessions in Geneva aimed at finetuning the deal.
Zarif, also Iran's top nuclear negotiator, said on Facebook Wednesday that "the nuclear talks are continuing with seriousness and a strong political will".
But some observers said a new generation of nuclear centrifuges, which could potentially enable Iran to rapidly purify uranium to a weapons-grade 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.
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.
"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.
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 Rouhani, who succeeded him last August.