updated 9/25/2009 3:41:06 PM ET 2009-09-25T19:41:06

Western intelligence puts Iran's newly revealed nuclear plant in the arid mountains southwest of Tehran, not far from one of the holiest cities in Shiite Islam.

Neither Iran nor the International Atomic Energy Agency has confirmed the location or size of the facility. Nor have they given details on its purpose. But diplomats with access to Western intelligence say the plant is about 160 kilometers — 100 miles — from the capital, near Qom, a center of Shiite religious teaching and the site of one of Shiism's most revered shrines.

The U.S. and Israel have not ruled out the possibility of a military strike on Iran's nuclear facilities as a last resort if Iran's continues to flout U.N. Security Council demands that it cease uranium enrichment.

But any strike near Qom would likely provoke a backlash among Shiite Muslims across the Middle East.

Iranian President Mahmoud Ahmadinejad said President Obama would regret his statements on the enrichment facility.

The intelligence assessment cited by diplomats says the site is meant to house no more than 3,000 enriching centrifuges — much less than the more than 8,000 machines at Iran's Natanz site which is being monitored by the IAEA.

Speculation on plant's sophistication
But the plant, which intelligence reports say is set to start operation next year, could be set up for advanced domestically developed centrifuges that enrich uranium at much higher speed and efficiency than the decades old P-1 type centrifuges acquired on the black market and enriching at Natanz.

That means Iran could enrich much more quickly with fewer centrifuges than at Natanz, where it has already accumulated enough low enriched material to turn out enough weapons-grade uranium — enriched to 90 percent and beyond — for one nuclear weapon.

Iran kept the facility hidden from weapons inspectors until a letter it sent to the IAEA on Monday.

Officials said the letter contained no details about the location of the second facility, such as when — or if — it had started operations or the type and number of centrifuges it was running.

But one of the officials, who had access to a review of Western intelligence on the issue, said it was underground about 100 miles southwest of Tehran. It is not yet operational but the U.S. believes it will be by next year, said a U.S. counterproliferation official, who spoke on condition of anonymity because he was not authorized to speak publicly.

Size of the heavily-guarded site was described as too small for commercial use, but an appropriate size for making weapons-grade uranium, enough to make a bomb or two a year, senior White House officials told NBC News on the condition of anonymity.

U.S. intelligence believes the facility is on a military base controlled by Iran's Revolutionary Guards, according to a document that the Obama administration sent to U.S. lawmakers. It was provided to The Association Press by an official on condition of anonymity because, though unclassified, it was deemed confidential. The military connection could undermine Iran's contention that the plant was designed for civilian purposes.

Official: Visits to facility OK
Tehran insists the facility is not a threat. Iranian nuclear chief Ali Akbar Salehi suggested Friday that International Atomic Energy Agency inspectors will be allowed to visit the previously secret facility.

Salehi, speaking only hours after the existence of the letter was revealed by diplomats, said the facility is a "successful new step in the direction of preserving and enjoying its accepted right for peaceful use of nuclear energy."

"The activities of this facility, like other nuclear facilities in Iran, will be in the framework of the measures of the agency," he said, suggesting that the new facility could be opened to inspectors, like Iran's known enrichment facility, Natanz.

But the fact that Iran disclosed the plant's existence only a few days before it was publicly revealed suggests it may have done so only because it wanted to go on record before being exposed.

At the G-20 summer in Pittsburg, Obama and the leaders of France and Britain declared that the secret nuclear facility puts new pressure on Tehran to quickly disclose all its nuclear efforts — including any moves toward weapons development — "or be held accountable."

Iranian President Mahmoud Ahmadinejad responded that his nation was keeping nothing from international inspectors and needn't "inform Mr. Obama's administration of every facility that we have."

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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