What do we know about what happened early on Sunday at Natanz, the nerve centre of Iran's atomic programme?

And how will it impact the talks in Vienna aimed at salvaging the 2015 nuclear agreement between Iran and world powers?

- What happened at Natanz? -

Ascertaining this is fraught with pitfalls as Iranian authorities have offered contradictory narratives and released no post-incident photos or video footage.

The Atomic Energy Organization of Iran said a "small explosion" had hit the plant's electricity distribution centre, causing damage that it said would be quickly repaired.

Iran's foreign ministry meanwhile accused Israel of an act of "terrorism" and vowed revenge.

Iranian government spokesman Ali Rabiei said Tuesday the "sabotage occurred in a duct of power cables leading to the centrifuge machines which caused damage to this system".

"This was not an external attack and the location of the sabotage has been clearly determined."

Israel has neither confirmed nor denied involvement but public radio reports said it was a sabotage operation by the Mossad spy agency, citing unnamed intelligence sources.

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".

- Why is Natanz important? -

Natanz is a key centre for Iran's nuclear programme and is kept under high security.

The 2015 agreement between Iran, the five UN Security Council permanent members and Germany placed key restrictions on the Islamic republic's nuclear programme in return for lifting sanctions.

It capped uranium enrichment -- measured by the presence of fissile isotope uranium-235 -- at 3.67 percent.

It also limited the number of so-called first-generation centrifuges, the only type permitted.

But Tehran has been producing uranium enriched up to 20 percent since January, one of the steps away from its commitments since Washington withdrew from the agreement in 2018 and reimposed 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, Rouhani had on Saturday also inaugurated a new centrifuge assembly factory at Natanz.

- What's the wider impact? -

In Iran, conservatives have vociferously attacked Rouhani's moderate reformist government and demanded that talks in Vienna be ditched.

The talks, which seek to bring the US back into the deal and persuade Iran to re-embrace its commitments, had got underway on April 6 on a positive note.

The negotiations involve the remaining parties -- Iran, Britain, China, France, Germany, and Russia -- with the EU acting as an intermediary between Washington and Tehran.

Iranian state television announced Tuesday that deputy foreign minister Abbas Araghchi had left for Vienna ahead of resumed talks originally scheduled for Wednesday.

Iran then announced that it would begin to enrich uranium to 60 percent.

Eric Brewer, a senior fellow at Washington's Center for Strategic and International Studies, said the 60-percent target was "a significant... step" towards shortening Iran's breakout timeline to a weapons capability.

- How about the Vienna talks? -

The events at Natanz represented "not only sabotage of centrifuges" but a "sabotage of diplomacy", said the head of arms proliferation at the Geneva Centre for Security Policy, Marc Finaud.

Russia's ambassador to the UN said late Tuesday that the resumption of the Vienna talks had been postponed by one day to Thursday.

The White House said it remained committed to negotiations with Iran despite Tehran's "provocative" statement that it will ramp up uranium enrichment.

"We are certainly concerned about these provocative announcements," President Joe Biden's press secretary, Jen Psaki, told reporters Tuesday.

"We believe that the diplomatic path is the only path forward here and that having a discussion, even indirect, is the best way to come to a resolution."

The Iranian president said the 60 percent target was a legitimate response to the attack on Natanz.

"What you did was nuclear terrorism," Rouhani said Wednesday, addressing Iran's arch-foe Israel. "What we do is legal."

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