BRUSSELS/VIENNA: Talks on rescuing Iran's 2015 nuclear deal will carry on for several days before breaking so that Iranian and US officials can return home for consultations, a European Union official said on Friday,

The EU is chairing meetings in Vienna of the remaining parties to the deal - Iran, Russia, China, France, Germany and Britain. A delegation from the United States, which pulled out of the accord under President Donald Trump, is based in a nearby hotel as Iran has refused to hold direct talks.

A second round of talks, which involve discussions in various formats as well as formal meetings of all the remaining parties, started on Thursday. The aim is a US return to the deal, lifting sanctions that were reimposed after its pullout, and undoing Iranian breaches of its nuclear restrictions.

Talks will continue "for a few days and then I think the two most relevant delegations will go back home to receive more precise instructions and then, I don't know when, we will resume," the EU official told reporters in a phone briefing.

The talks have been overshadowed by an explosion at Iran's main uranium-enrichment facility at Natanz, which Tehran has blamed on Israel, and Iran's decision to enrich uranium to 60% purity, a big step towards weapons-grade, which it said it started doing on Friday.

"We have this (Iranian) decision to go for 60% enrichment. Obviously this is not making the negotiation easier," the official said, calling what happened at Natanz "deliberate sabotage". It is not clear how long the talks will last in total, he added.

Israel - widely believed to be the only Middle Eastern country with a nuclear arsenal - has not formally commented on the Natanz incident. Several Israeli media outlets have quoted intelligence sources as saying the country's Mossad spy service carried out the operation.