Iran said Saturday it will allow U.N. nuclear inspectors to examine its newly revealed, still unfinished uranium enrichment facility as world criticism mounted over the underground site that was developed secretly.

The presence of a second uranium-enrichment site that could potentially produce material for a nuclear weapon has provided one of the strongest indications yet that Iran has something to hide _ despite its repeated assertions that its program is only to generate electric power.

That impression was reinforced by a close aide to Supreme Leader Ayatollah Ali Khamenei, who said the site will be operational "soon" and would pose a threat to those who oppose Iran.

"This new facility, God willing, will become operational soon and will blind the eyes of the enemies," Mohammad Mohammadi Golpayegani told the semi-official Fars news agency.

The existence of the secret site was first revealed by Western intelligence officials and diplomats on Friday. It is located in the arid mountains near the holy city of Qom, inside a heavily guarded, underground facility belonging to Iran's elite Revolutionary Guards, according to a document sent by the Obama administration to lawmakers.

The revelation of the secret site has given greater urgency to a key meeting on Thursday in Geneva between Iran and six major powers trying to stop its suspected nuclear weapons program.

The U.S. and its partners plan to tell Tehran at the meeting that it must provide "unfettered access" for the International Atomic Energy Agency to its previously secret Qom enrichment facility within weeks, a senior Obama administration official said Saturday in Washington.

The U.S., Britain, France, Germany, China and Russia also will present in the meeting a a so-called transparency package _ including access to scientists, documents and computers _ covering all of Iran's nuclear activities across the country, the official said. The official spoke on condition of anonymity in order to discuss plans that are not yet ready to be announced.

The six powers will demand that Iran prove to the increasingly skeptical group that its intentions with its various sites are peaceful and energy-related, as Iran claims, and not for weapons development, as the West believes, the official said Saturday.

These nations now agree that they are less inclined to listen to suspect arguments or incomplete evidence _ viewing it as a stall tactic, the official said.

But beyond the timeframe of "weeks" for coming clean on Qom, the six countries will not give Iran a specific deadline to provide the information about its overall program, the official said.

The development of such a timeframe will depend on the Iranians' actions in the meeting and directly after it, the official said.

Earlier Saturday, President Barack Obama in his weekly radio and Internet address offered Iran "a serious, meaningful dialogue" over its disputed nuclear program, while warning Tehran of grave consequences from a united global front.

Evidence of the clandestine facility was presented Friday by Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy at the G-20 economic summit in Pittsburgh.

Israeli Foreign Minister Avigdor Lieberman said the revelation was firm proof Iran was seeking nuclear weapons.

"This removes the dispute whether Iran is developing military nuclear power or not and therefore the world powers need to draw conclusions," Lieberman told Israel radio. "Without a doubt, it is a reactor for military purposes not peaceful purposes."

Israel considers Iran a strategic threat due to its nuclear program, missile development and repeated references by President Mahmoud Ahmadinejad to Israel's destruction.

Vice President Ali Akbar Salehi, who heads Iran's nuclear program, said on national television that inspectors from the IAEA could visit the site, though he did not specify when.

Salehi said there was nothing secret about the site and that Iran complied with U.N. rules that require it to inform the world body's nuclear agency six months before a uranium enrichment facility becomes operational.

"Under (NPT) rules, we are required to inform the IAEA of the existence of such a facility 180 days before introducing materials but we are announcing it more than a year earlier," he said.

The Iranians claim to have withdrawn from an agreement with the IAEA requiring them to notify the agency of the intent to build any new nuclear facilities and instead are now only subject to the six-month notification requirement before a facility becomes operational.

But the IAEA says Tehran cannot unilaterally withdraw from that bilateral agreement and still should have announced its plans to build the facility.

The statement by Khamenei's aide that the facility will be operational "soon" seemed to suggest that it could be ready even ahead of the 18-month figure cited by Salehi.

The small-scale site is meant to house no more than 3,000 centrifuges _ much less than the 8,000 machines at Natanz, Iran's known industrial-scale enrichment facility, but they could still potentially help create bomb-material.

Experts have estimated that Iran's current number of centrifuges could enrich enough uranium for a bomb in as little as a year. Washington has been pushing for heavier sanctions if Iran does not agree to end enrichment.

U.N. Secretary General Ban Ki-moon expressed grave concern about the facility and said "the burden of proof is on Iran," in a statement by his office released after he met privately with Ahmadinejad Friday night.

Salehi said construction of the Qom facility was a "precautionary measure" to protect Iran's nuclear facilities from possible attacks.

"Given the threats we face every day, we are required to take the necessary precautionary measures, spread our facilities and protect our human assets. Therefore, the facility is to guarantee the continuation of our nuclear activities under any conditions," he told the television.

Hans Blix, the former chief U.N. weapons inspector in Iraq, however, told Sky News television that Iran was clearly bucking the international community's demand that it keep its activities transparent.

Blix, speaking from Stockholm, told the broadcaster that the clerical regime was clearly going against the spirit of the international community's demands.

"The revelation of a second plant shows that they are not exactly transparent, as the IAEA has asked them to be," he said. "This has not been an exercise in openness."

If Iran were developing nuclear weapons, it would be at precisely such a place, noted Mark Fitzpatrick, of the London-based International Institute for Strategic Studies.

"If they were to develop a nuclear weapon they would probably do it at a clandestine facility so that they wouldn't trigger the obvious trip wire," he said.

While an actual weapon is several years away, said Paul Rogers a security expert at the University of Bradford in northern England, something rudimentary could be rigged up within a year if Iran stepped up its enrichment activity.

"If it were to re-enrich its low-grade uranium to weapons-grade for an experimental device it would take about a year," he said. "That would almost be a demonstration to show it had the capability."

The key Western powers at the United Nations have given Tehran until year's end to cease enriching uranium or face new sanctions.

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Associated Press Writer Raphael G. Satter contributed to this report from London.