Prime Minister Binyamin Netanyahu accused Iran on Friday of making "accelerated progress towards achieving nuclear weapons", adding that it was "totally ignoring" Western demands to rein in its atomic program.
Netanyahu made the remarks to a visiting US congressman the day after diplomatic sources told Reuters that Iran had installed more uranium enrichment machines in an underground bunker, potentially paving the way for a significant expansion of its nuclear work.
"Only yesterday we received additional proof that Iran is continuing accelerated progress towards achieving nuclear weapons and is totally ignoring international demands," Netanyahu was quoted as saying by his office.
Meanwhile, in Vienna, the UN nuclear watchdog pressed Iran to address suspicions about nuclear bomb research in the Islamic state, part of diplomatic efforts to resolve the dispute before any possible military action by Israel or the United States.
A flurry of bellicose rhetoric from some Israeli politicians this month has ignited speculation that Israel might strike nuclear sites before the US presidential election in November.
On the eve of Friday's talks between Iran and the United Nations' International Atomic Energy Agency (IAEA), diplomatic sources told Reuters that Iran had installed many more uranium enrichment centrifuges at its Fordow underground facility.
Though the new machines are not yet operating, the move signalled Tehran's defiance of international demands to suspend enrichment and may strengthen the Israeli belief that toughened sanctions are failing to make Tehran change course.
The sources also said satellite imagery indicated Iran had used a brightly colored, possibly pink, tent-like structure to cover a building at a military site which the UN watchdog wants to inspect, raising new concerns about suspected cleansing of evidence of illicit past nuclear work there.
Iran, the Jewish state's arch-enemy and a major oil producer, denies it is seeking nuclear weapons and has threatened wide-ranging reprisals if attacked.
Iran's envoy to the IAEA, Ali Asghar Soltanieh, said he expected progress in Friday's meeting: "Both sides are trying to bridge the gap," he told reporters at Iran's mission in Vienna.
Chief UN inspector Herman Nackaerts, in charge of the agency's long-stalled inspection effort, said the aim was to reach an agreement on how to resolve the IAEA's questions about possible military dimensions to Iran's nuclear program.
Nackaerts, who headed the IAEA delegation, said he would reiterate his request for access to the Parchin military facility, where he believes Iran has undertaken explosives tests relevant for developing a nuclear weapons capability.
Their meeting is separate from Tehran's negotiations with world powers that have made little headway since they resumed in April after a 15-month hiatus, but the focus on suspicions about Iran's nuclear ambitions mean they are still closely linked.
Washington says there is still time for diplomatic pressure to work in making Iran curb its nuclear enrichment program, which is the immediate priority of the six powers - the United States, Britain, Russia, China, France and Germany. Refined uranium can have both civilian and military purposes.
IAEA chief not optimistic
Iran says it seeks only civilian nuclear energy.
But its refusal to limit and open up its atomic activity to unfettered IAEA inspections that could determine whether it is purely peaceful or not has led to tightening punitive measures and speculation about Israeli or US military action.
Western diplomats expect no major breakthrough on Friday but say Iran could offer a concession to UN inspectors seeking access to sites, officials and documents in an attempt to blunt their upcoming quarterly report on Iran, which is due next week.
Such a move would likely be intended to undermine a planned Western push to rebuke Iran at an IAEA board meeting next month over its failure to cooperate with the agency's inquiry, and should be treated with skepticism, one envoy said.
The IAEA's immediate priority is to gain prompt access to Parchin, even though Western diplomats say it may now have been purged of any evidence of nuclear weapons research, possibly carried out a decade ago.
Citing satellite images, the diplomats say Iran has demolished some small buildings and moved earth at Parchin.
On Thursday, diplomatic sources said the building believed to be housing an explosives chamber - if it is still there - had been "wrapped" with scaffolding and tarpaulin, hiding any sanitization or other activity there from satellite cameras.
Iran says Parchin, about 30 km (20 miles) southeast of the capital Tehran, is a conventional military site and has dismissed allegations aired about it as "ridiculous". It says a broad framework agreement for how the IAEA should conduct its probe is needed before possibly allowing access to Parchin.
IAEA Director General Yukiya Amano this week said the agency would pursue access to Parchin, even though the suspected sanitisation would probably "hamper our verification activities," if and when inspectors can go there.