After a positive first round of negotiations aimed at resurrecting the 2015 agreement scuttled by Donald Trump, Iran's push towards enrichment levels needed for military use "puts pressure on everyone," a European diplomat told AFP.

Britain, France and Germany have expressed "grave concern" over the most recent enrichment announcement, while also rejecting "all escalatory measures by any actor".

Iranian President Hassan Rouhani on Thursday re-stated the country's long-standing position that "we are not seeking to obtain the atomic bomb" and that it was a "mistake" for Europe and United States to express concern that the country could enrich uranium to weapons-grade levels "in one go".

Tehran says the enrichment move is a response to Israel's "nuclear terrorism" after an explosion on Sunday knocked out power at its Natanz enrichment plant.

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

"It definitely complicates things," the diplomat said, ahead of the talks between the remaining members of the deal -- Germany, France, Britain, China, Russia and Iran -- resuming at 12:30 pm local time (1030 GMT).

US Secretary of State Antony Blinken said late Wednesday that Washington was taking the "provocative announcement" on enrichment from Iran "very seriously".

"I have to tell you the step calls into question Iran's seriousness with regard the nuclear talks," Blinken told reporters in Brussels.

But events of the past few days have also "reminded both parties that the status quo is a lose-lose situation", and have "added urgency" to the talks, said Ali Vaez, Iran Project Director at the International Crisis Group think tank.

"It is clear that the more the diplomatic process drags on, the higher the risk that it gets derailed by saboteurs and those acting in bad faith," Vaez added.

- 'Only viable solution' -

Known as the Joint Comprehensive Plan of Action (JCPOA), the deal has been disintegrating since former US president Donald Trump dramatically withdrew from it in 2018 and re-imposed sanctions, prompting Iran to retaliate by exceeding its agreed limits on nuclear activity.

Russia's representative in Vienna said the deal remained the "only viable solution which can bring the Iranian nuclear programme back to the agreed parameters."

But the Joe Biden administration, while agreeing on the JCPOA's value, has stressed that it is waiting for Iran to first roll back steps away from compliance that it took to protest Trump's sanctions.

An American delegation is attending the talks "indirectly", staying at a separate hotel.

Washington is "very open-eyed about how this will be a long process," White House press secretary Jen Psaki told reporters Wednesday.

"It's happening through indirect discussions, but we still feel that it is a step forward."

- 'Dangerous spiral' -

In the meantime, Tehran is reducing its "breakout time" -- time to acquire the fissile material necessary for the manufacture of a bomb -- the European diplomat said.

Under the JCPOA, it had committed to keep enrichment limited to 3.67 percent, though it stepped this up to 20 percent in January.

The UN's International Atomic Energy Agency said its inspectors visited the site at Natanz for "verification and monitoring activities" on Wednesday, and that Iran had "almost completed preparations" to enrich uranium to 60 percent purity.

Its foreign minister Mohammad Javad Zarif said the Natanz attack had unleashed a "dangerous spiral" and warned Biden the situation could only be contained by lifting the sanctions Trump imposed.

"It was unrealistic to expect Iran not to respond to such a humiliating attack at the heart of its nuclear programme," the ICG's Vaez said.

"But the only thing that in the past two decades has effectively curtailed Iran's nuclear programme has been diplomacy, not sanctions or sabotage."