Israel reportedly shared intelligence with the U.S. and European allies suggesting that Iran is preparing to enrich uranium to 90 percent purity - the level needed for nuclear weapons - in the run-up to the resumption of talks on Monday.
Diplomats say time is running out to save the 2015 Iran nuclear pact, which then-U.S. President Donald Trump abandoned in 2018.
But U.S. sources told the Axios news website that Israel believes Iran could be ready to produce 90 percent enriched uranium within weeks.
Israeli Defense Minister Benny Gantz said on Monday that Israel had shared 'intelligence which points to Iran’s continued race toward a nuclear weapon while violating the 2015 agreement.'
And Israeli Foreign Minister Yair Lapid, who met with his British counterpart on Monday, said Iran was intent on continuing with its nuclear program whatever happened at talks in Vienna.
'They will play for time, earn billions from the removal of sanctions, continue to deceive the world, and covertly advance their nuclear program,' he tweeted.
'This is what they have done in the past, and it is what they will do this time as well.'
Talks between signatories to the 2015 Iran nuclear deal resumed at a hotel in Vienna on Monday. Israel reportedly warned allies during the run-up that Iran was preparing to enrich uranium to 90 percent purity - the level needed for nuclear weapons
Former President Donald Trump withdrew the U.S. from the deal in 2018, accusing Iran of breaching the spirit of the agreement by testing ballistic missiles and supporting proxies
Israel's Prime Minister Naftali Bennet has called on allies to reject the Iran nuclear deal
Iran has previously claimed that it could reach 90 percent enrichment if it wanted to.
Iran stuck to the agreement for a year after Washington withdrew, but since then has built new centrifuges banned under the deal and breached enrichment limits, according to the International Atomic Energy Agency.
Talks at bringing Iran back into line and the U.S. back to the deal resumed in Vienna on Monday. They had been suspended for five months after the election of a hard-line government in Tehran.
Diplomats from Iran and the five countries still party to the Joint Comprehensive Plan of Action - China, France, Germany, Russia and the UK - met at a hotel in the Austrian capital.
The U.S. took part indirectly, with European officials shuttling back and forth.
The European Union official chairing the talks offered an optimistic assessment.
'I feel positive that we can be doing important things for the next weeks,' E.U. diplomat Enrique Mora told reporters.
He added that the new Iranian delegation had stuck to its demand that all sanctions be lifted. But he also suggested Tehran had not rejected outright the results of the previous six rounds of talks held between April and June.
Israel's Foreign Minister Yair Lapid said Iran was intent on using talks to lift sanctions while 'covertly' advancing its nuclear program
'They have accepted that the work done over the first six rounds is a good basis to build our work ahead,' he said.
'We will be of course incorporating the new political sensibilities of the new Iranian administration.'
Iran has repeatedly said its nuclear program has a civilian rather than military purpose, and that progress on talks cannot be made unless the U.S. drops the sanctions reimposed by Trump.
Writing in the Financial Times ahead of the latest round of talks, Iran's new negotiator Ali Bagheri Kani said: 'From our viewpoint, past blunders should not be repeated. We have all, respectively, learned over the past six years what and who can be trusted.
'To ensure any forthcoming agreement is ironclad, the west needs to pay a price for having failed to uphold its part of the bargain. As in any business, a deal is a deal, and breaking it has consequences.'
For his part, President Joe Biden had made clear he is willing to lift sanctions if Iran makes the first move in reversing course.