Iran plans to sharply increase the purity of its enriched uranium in a provocative move that puts it closer to weapons-grade levels and threatens to scupper efforts to revive an international nuclear deal.

Tehran on Tuesday said it had informed the UN nuclear watchdog of its decision to as much as triple enrichment of the radioactive metal to up to 60 per cent.

Iranian media reported the country’s move was in response to an alleged foreign attack last weekend on the Natanz facility, its largest uranium enrichment site.

Tehran has suggested the incident, which caused a blackout and is believed to have done major damage, was Israeli sabotage. Israel has not confirmed or denied responsibility.

“Iran will add 1,000 more centrifuges which are 50 per cent stronger than the existing machines to the Natanz site on top of replacing the ones damaged [in the incident],” said Abbas Araghchi, Iran’s deputy foreign minister.

The announcement came as negotiators from the nuclear agreement’s remaining signatories — Iran, the EU, Germany, France, the UK, Russia and China — prepared to resume talks in Vienna to restore the 2015 accord and pave the way for the US to rejoin the deal.

President Joe Biden has said the Washington is prepared to rejoin the agreement, which Donald Trump abandoned in 2018, if Iran comes back into compliance with it.

Tehran’s enrichment move will stoke anxieties in European capitals about prospects for the talks on the accord, under which Iran accepted curbs on its nuclear programme in exchange for the lifting of many international sanctions.

One European diplomat branded the announcement a “maximum provocation” aimed at gaining leverage. It was “too early to say” what impact it would have on the negotiations, the official added.

The Trump administration imposed tough sanctions on Iran while Tehran retaliated with an escalating series of breaches of the accord’s limits, including previously increasing the purity of its enriched uranium from the 3.67 per cent agreed under the accord to 20 per cent.

Diplomats said the Vienna talks made progress last week but many obstacles remain, including the sequencing of moves by Washington and Tehran.

Iran’s supreme leader, Ayatollah Ali Khamenei, has set what many regard as an impossible condition to roll back the country’s nuclear advance: he wants the US to lift sanctions first and give Tehran time to verify that.

An Iranian regime insider said the US “has no intention to lift the sanctions and is only [playing] a game with Iran while the Islamic republic has no intention to back off either”.

“Israel is in the meantime trying to dismantle Iran’s nuclear programme through sabotage,” the insider said.

Israeli media have cited unnamed Israeli officials alluding to the Jewish state’s role in the Natanz incident.

Israel’s prime minister Benjamin Netanyahu has been one of the most vociferous critics of the nuclear deal Tehran signed with world powers in 2015, and backed Trump’s decision to pull the US from the accord and impose crippling sanctions on the republic.

Netanyahu has made it clear he opposes any rapprochement by Biden.

In recent months Israel and Iran have traded accusations as they engage in a shadow war.

In November, Iran accused Israel of killing its top nuclear scientist Mohsen Fakhrizadeh. Israel’s intelligence minister said at the time of the killing that he did not know who was behind the incident.

The Islamic republic and the Jewish state also have accused each other of attacking their respective container ships.

Iranian media said an Israeli vessel off UAE waters was attacked on Tuesday, without giving many details. In February, Israel alleged that Iran targeted an Israeli-owned ship in an explosion in the Gulf of Oman. Iran has denied involvement.

A month later, Tehran said an Iranian container ship was targeted by a “terrorist attack” in the Mediterranean Sea, and in April another vessel was hit by an explosion in the Red Sea.

Iran’s enrichment announcement came just hours after Russia accused the EU of undermining efforts to revive the nuclear deal by imposing sanctions on Tehran officials over alleged serious human rights abuses.

Sergei Lavrov, Russia’s foreign minister, on Tuesday denounced as “worse than a crime” an EU move to slap travel bans and asset freezes on eight Iranian security force commanders over a deadly 2019 crackdown on protesters.