Iran has accused its nemesis Israel of sabotaging a nuclear enrichment facility purportedly hit by an explosion a day after President Hassan Rouhani inaugurated advanced uranium centrifuges.

What do we know about what happened early on Sunday at Natanz, the nerve centre of Iran's atomic programme? And is dialogue scheduled to resume in Vienna aimed at salvaging the 2015 nuclear agreement between Iran and world powers still viable?

Ascertaining this is fraught with pitfalls, not least when Iranian authorities have at times offered contradictory narratives, having initially described Sunday's incident as an "accident".  Also, while keen to display Saturday's inauguration of more powerful centrifuges, Iranian media have not published any post-incident video images or photographs of the site.

The "sabotage occurred in a duct of power cables leading to the centrifuge machines which caused damage to this system", government spokesman Ali Rabiei said Tuesday. "This was not an external attack and the location of the sabotage has been clearly determined," he added.

The Atomic Energy Organization of Iran (AEOI) said the previous day a "small explosion" had hit the plant's electricity distribution centre, causing damage that would be quickly repaired. But Iran's foreign ministry accused Israel of an act of "terrorism" and vowed vengeance at a time and place of Tehran's choosing. 

The New York Times, quoting unnamed US and Israeli intelligence officials, also said there had been "an Israeli role" in the attack. The paper said an explosion had "completely destroyed" the power system that fed the site's "underground centrifuges". Washington denied any involvement. The Natanz nuclear site is a key centre for Iran's nuclear programme and is kept under extremely high security.

The 2015 nuclear agreement between Iran, the five permanent UN Security Council powers and Germany placed key restrictions on the Islamic republic's nuclear programme in return for lifting sanctions. But Iran has been producing uranium enriched up to 20 per cent since January, adding to earlier steps away from its commitments in response to the US in 2018 pulling out of the deal and reimposing biting sanctions.

The alleged sabotage took place less than 24 hours after Iran announced it had brought into service or begun testing several hundred centrifuges forbidden under the 2015 agreement.

In July 2020, an advanced centrifuge assembly factory at Natanz was badly damaged by a mysterious explosion, likewise described as a "terrorist" act by Tehran.  

Alongside the new centrifuges themselves, Rouhani had on Saturday inaugurated a new centrifuge assembly factory at Natanz.

In Iran, conservatives have vociferously attacked Rouhani's moderate reformist government and demanded that dialogue in Vienna aimed at rescuing the 2015 deal be ditched. Seeking to bring the US back into the deal and persuade Iran to re-embrace its commitments, talks in the Austrian capital had got underway on April 6 on a positive note.

The negotiations involve the remaining parties -- Iran, China, Russia, Germany, the UK and France -- with the EU acting as an additional go-between between Washington and Tehran.  Iranian state television announced Tuesday afternoon that deputy foreign minister Abbas Araghchi had left for Vienna ahead of a resumption of talks planned for Wednesday.

But early in the evening, Iran announced that it would begin to enrich uranium to 60 per cent.  Eric Brewer, a senior fellow at Washington's Center for Strategic and International Studies, noted on Twitter that the 60 per cent target was "a significant... step" towards further shortening Iran's breakout timeline.

Indeed, it would quickly allow Iran to reach the 90 per cent threshold needed for military purposes, even if it has repeatedly insisted it does not seek an atomic bomb. Russia's ambassador to the UN in Vienna said late Tuesday that a resumption of talks had been postponed for one day, to Thursday. 

After voicing "concerns" in January over Iran's decision to enrich uranium up to 20 per cent, the Europeans and Washington in February urged Tehran not to disengage further. For Brewer, enriching to 60 per cent "is... unlikely to have the intended effect of forcing the US to accept Iran's demands. "So, buckle up" for a bumpy diplomatic ride, he forecast.

On Monday, Marc Finaud, head of Arms Proliferation at the Geneva Centre for Security Policy, said events at Natanz represented "not only sabotage of centrifuges," but a "sabotage of diplomacy".