Iran said Saturday it will allow UN 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 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.

US President Barack Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy accused Iran on Friday of constructing a secret underground uranium enrichment facility and of hiding its existence from international inspectors for years.

Just hours before Iran said it would allow International Atomic Energy Agency inspections of the site, Obama warned Tehran of grave consequences from a united global front and offered "a serious meaningful dialogue" over its nuclear program.

The White House responded to Iran's offer of inspections by urging Iran's complete and immediate cooperation with the IAEA.

Obama said in his address that evidence of Iran's building the underground plant "continues a disturbing pattern of Iranian evasion" that jeopardizes global nonproliferation.

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 UN 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, which many nations believe is part of Tehran's drive to build a nuclear weapon. Iran says its nuclear program is designed to generate electricity.

UN 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.

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 UN 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.

On Oct. 1 diplomats from Iran, the US, Britain, France, Russia, China and Germany will meet to discuss Tehran's nuclear program.