The announcement cast a shadow over ongoing talks in Vienna aimed at salvaging the tattered 2015 nuclear deal between Iran and world powers that former US president Donald Trump abandoned three years ago.

Tehran has written to the UN's International Atomic Energy Agency to announce "that Iran will start 60 percent enrichment," the official IRNA news agency reported.

The move will bring Iran closer to the 90 percent purity threshold for military use and shorten its potential "breakout time" to build an atomic bomb, a goal it denies.

Under the nuclear deal, Iran had committed to keep enrichment to 3.67 percent, though it had stepped this up to 20 percent in January.

The latest news came two days after an explosion knocked out power at Iran's main nuclear facility of Natanz in central Iran which the Islamic republic blamed on Israel and labelled an act of "terrorism".

Israel, which did not claim responsibility, is strongly opposed to US President Joe Biden's efforts to revive the nuclear agreement.

The accord between Iran and the UN Security Council's permanent members plus Germany promised Iran relief from punishing sanctions in return for agreeing to limits on its nuclear programme.

Israel has vowed it will stop the Islamic republic from ever building an atomic bomb, which it considers an existential threat to the Jewish state.

- 'Bad gamble' -

The mysterious blast at Natanz has sharply heightened tensions between the two powers already engaged in a shadow war on lands and seas across the Middle East, with Iran on Monday vowing to take "revenge".

"If (Israel) thought that they can stop Iran from following up on lifting sanctions from the Iranian people, then they made a very bad gamble," Iranian Foreign Minister Mohammad Javad Zarif warned.

Iran would make the enrichment plant "more powerful" by using advanced centrifuges, he added.

IRNA reported that Iran would also add "1,000 centrifuges with 50 percent more capacity to the machines present in Natanz, in addition to replacing" those damaged in the attack.

According to Iran's English-language channel Press TV, the enrichment jump will start on Wednesday.

Zarif, after talks with his visiting Russian counterpart Sergei Lavrov, also warned Israel's ally the United States it would gain no extra leverage in Vienna through "acts of sabotage" and sanctions.

The White House has denied all US involvement in the Natanz incident.

Unsourced Israeli media reports attributed the blast to Israeli security services.

The New York Times, quoting unnamed US and Israeli intelligence officials, also said there had been "an Israeli role" in the attack in which an explosion had "completely destroyed" the power system that fed the plant's "underground centrifuges".

Quoting another unnamed intelligence source on Tuesday, the NYT added that an "explosive device had been smuggled" into the site and "detonated remotely", taking out primary and backup power.

- 'Worse than a crime' -

Lavrov, during his Tehran visit, stressed Russian support for Iran's position.

"We are counting on the fact that we will be able to save the agreement and that Washington will finally return to full and complete implementation of the corresponding UN resolution," Lavrov said.

Europe's "inability to implement" its nuclear deal commitments and "bowing to America's pressure" shows it is "slowly losing its relevance in international relations", Zarif said.

He also blasted the European Union for slapping sanctions on eight Iranian security officials, in response to a crackdown on 2019 street protests, saying the blacklisting threatens efforts to restore the deal.

"If this decision was taken voluntarily in the midst of negotiations in Vienna... it is a mistake worse than a crime," said Lavrov.

"I hope our European colleagues understand that such actions are unacceptable and will take measures not to allow the talks to be derailed."

Lavrov's remarks come at a time of heightened tensions between Russia and the West over various issues, also including Ukraine.

Iranian President Hassan Rouhani meanwhile told Lavrov that Iran expects a "return to 2015's agreements and obligations".

For now, the agreement remains in limbo with neither Tehran nor Washington backing down from their positions, and each demanding the other make the first move.

Ali Vaez, analyst with the International Crisis Group, said Iran's vow to step up enrichment showed the long-running "maximum pressure" campaign of Trump and Netanyahu had backfired.

"Thanks to Netanyahu and max pressure cheerleaders in DC, Iran's nuclear program is now going to be enriching at unprecedented levels, with less international oversight," he wrote on Twitter. "Tremendous work, everyone!"

amh/mj/fz/dv

THE NEW YORK TIMES COMPANY