The remaining parties to the faltering 2015 Iran nuclear accord met Wednesday after Tehran announced plans for a new breach of the deal, and as uncertainty reigns ahead of US President-elect Joe Biden's January inauguration.
The meeting of the so-called "joint commission" includes China, France, Russia, Iran, Germany and Britain and is chaired by senior EU foreign affairs official Helga Schmid.
The meeting, which started at 11 am (1000 GMT), is being held virtually because of the coronavirus pandemic.
The 2015 deal, known as the Joint Comprehensive Plan of Action (JCPOA), has disintegrated steadily since US President Donald Trump withdrew from it in 2018 and went on to impose crippling economic sanctions on Iran.
Tehran has retaliated by progressively abandoning limits on its nuclear activity laid down in the deal, most recently planning to install advanced centrifuges at Iran's main nuclear enrichment plant in Natanz.
Last week France, Germany and Britain collectively known as the "E3" condemned the plan as "deeply worrying".
Meanwhile the assassination last month of prominent Iranian nuclear scientist Mohsen Fakhrizadeh has heightened tension in the region, with Iran blaming the killing on Israel.
In the wake of Fakhrizadeh's death, Iranian MPs passed a bill calling for further expansion to Iran's nuclear programme and an end to inspections of nuclear facilities by the UN watchdog International Atomic Energy Agency (IAEA).
The Iranian foreign ministry said it did not agree with the bill and President Hassan Rouhani has suggested he will not sign it into law.
Rouhani has defied criticism from Iran's ultra-conservatives to state his determination to seize the "opportunity" presented by the change of US president in January.
- 'Not the best moment' -
Rouhani has said Iran is ready to come back into compliance with the deal as soon as other parties fulfil their commitments.
President-elect Biden has said he is willing to return to the deal but has revealed little else about forthcoming US strategy on the question.
Before the start of Wednesday's talks, Russia's ambassador to international organisations in Vienna, Mikhail Ulyanov, tweeted that the focus would be on how to "preserve the nuclear deal and ensure its full and balanced implementation".
"The role of (the) US in this regard will inevitably be discussed," he added.
Another diplomat said the meeting would rather be "an opportunity to say to the Iranians face to face to stop the breaches of the deal" and not to ruin the chances of a return to diplomacy under Biden.
The meeting "is not coming at the best moment", the diplomat admitted, given the uncertainty over possible developments between now and Biden's January 20 inauguration.
Tensions between Tehran and the West have also been worsened in recent days by the execution in Iran last week of France-based dissident Ruhollah Zam, which provoked a global outcry.
But despite the various sources of friction, the diplomat said that inspections were continuing "as normal" on the ground.
On Monday EU foreign policy chief Josep Borrell said Wednesday's meeting was part of "our work in order to keep the JCPOA alive".
A few days previously he had emphasised that the deal was "the only way to avoid Iran becoming a nuclear power" and said a meeting of ministers from participants to the JCPOA would be called before Christmas.