Iran's uranium enrichment program appears stalled despite tough talk from the Tehran leadership, leaving intelligence services guessing about why the country has not made good on plans to press ahead with activities that the West fears could be used to make nuclear arms, diplomats said Thursday.

Outside monitoring of Iran's nuclear endeavors is restricted to International Atomic Energy Agency inspections of declared sites, leaving significant blind spots for both the agency and intelligence agencies of member countries trying to come up with the full picture.

Still, Tehran's reluctance to crank up activities at its declared enrichment site at Natanz when it seems to have the technical know-how for at least experimental work is puzzling the diplomatic and intelligence communities -- with some saying it was potentially worrisome.

Diplomats accredited or otherwise linked to the Vienna-based IAEA said some intelligence services believed the Natanz site is a front.

While the world's attention is focused on Natanz, Iranian scientists and military personnel could be working on a secret enrichment program at one or more unknown sites that is much more advanced, the diplomats, who demanded anonymity in exchange for discussing restricted information, said.

At the same time, though, they said the lack of new activity at the two pilot enrichment plants set up at Natanz could be good news.

The diplomats said that suggests possible Iranian hesitancy to provoke U.N. Security Council sanctions harsher than the relatively mild penalties agreed last month in response to Tehran's refusal to heed an August council deadline to suspend enrichment. Or, they said, it could be a sign of headway by relative moderates in the leadership unhappy with Iranian President Mahmoud Ahmadinejad's confrontational manner.

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Criticism has increased in the wake of municipal elections last month in which candidates associated with Ahmadinejad rivals made headway.

In an editorial reflecting disenchantment, the conservative Jomhuri Eslami newspaper on Tuesday warned the president that his choices of words "in nuclear issues is too aggressive," saying his language "implies that there is a kind of obstinacy in (the) nuclear case."

Anthony Cordesman, an Iran expert at the Center for Strategic and International Studies, suggested an additional possibility linked to theories that Tehran was forging ahead with its enrichment program elsewhere -- fear that any major progress at Natanz could provoke military action by Israel or the United States.

"It's a known facility and more and more of the subject of discussion as a possible Israeli or U.S. target," said Cordesman in a phone call from Washington. "So, do you use this facility now or wait to see what threat ... you face?"

IAEA inspectors arrived at Natanz on Wednesday for a routine round of monitoring. But one of the diplomats said they were unlikely to find anything but the status quo -- two small pilot plants assembled in 164-centrifuge "cascades" but working only sporadically to produce small quantities of non-weapons grade enriched uranium and other individual centrifuges undergoing mechanical testing. That has essentially been the situation at Natanz since late November, he said.

There have been no signs of any activity linked to plans to assemble 3,000 centrifuges at Natanz and move them into an underground facility as the start of an ambitious program foreseeing more than 50,000 centrifuges producing enriched material, said the diplomats.

Enrichment can result in either low-level nuclear fuel or the highly processed fissile core for nuclear weapons. Tehran insists it is interested only in generating power with the program.

Its refusal to suspend enrichment work led the U.N. Security Council to impose sanctions Dec. 23 -- relatively mild penalties banning specified materials and technology that could contribute to Iran's nuclear and missile programs. It also imposed an asset freeze on key companies and individuals in the country's nuclear and missile programs named on a U.N. list and gave the country 60 days to comply or face the likelihood of tougher nonmilitary sanctions.

Iran's leadership has shrugged off the U.N. moves, with Ahmadinejad and others vowing to expand the country's enrichment program and move quickly toward the first stage of industrial-scale enrichment -- the assembly of the 3,000 centrifuges.

Still, one of the diplomats said such a project is immensely complicated and would take months to complete even for countries with a developed enrichment program. While the underground facility at Natanz appears ready, not even preliminary assembly work has started there, they said.

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