"American thinking is coalescing around the suspicion that there are more Iranian nuclear sites that haven't yet been made public," Andrew Koch, the Washington correspondent of Jane's Defense Weekly, told AFP.

In a meeting in Vienna on Wednesday, the International Atomic Energy Agency (IAEA) had condemned Iran for 18 years of covert nuclear activities but stopped short of taking Tehran to the United Nations Security Council for possible sanctions.

The resolution was a compromise between the US call to censure Iran for hiding efforts to develop the capability to make atomic weapons and demands from Britain, France and Germany that Iran be rewarded for cooperating since October with the IAEA.

But diplomats and analysts said the United States was clearly calculating that it was only a matter of time before more Iranian violations of nuclear Non-Proliferation (NPT) safeguards would be discovered and set off an international uproar that would force the IAEA to take Iran to the UN's highest body in New York.

Koch said US intelligence officials believed there was "still another site somewhere" hidden by Iran, even though Tehran insists it has told all and there is nothing more to find in a nuclear program that is strictly for peaceful ends.

Koch said the information could come out in many different ways.

An Iranian resistance group had in August 2002 blown the whistle on secret Iranian activities when it revealed that the Islamic regime was building an underground uranium-enrichment plant at Natanz, 250 kilometres (150 miles) south of Tehran, and a heavy-water production facility at Arak, about 200 kilometres southwest of Tehran.

This revelation set off the current round of IAEA scrutiny of Iran's nuclear ambitions.

The IAEA has been inspecting Iran's program since February, and its director general Mohamed ElBaradei has issued three reports, with the latest saying Iran had hidden its production of small amounts of plutonium and enriched uraniam.

ElBaradei said the IAEA was now embarked on a "robust" series of inspections to verify that "nuclear activities in Iran are fully declared and are exclusively for peaceful purposes."

Iran has pledged to sign an additional protocol to the NPT that would allow for more extensive, snap inspections, and ElBaradei said Wednesday he was holding Tehran to its promise to already act as if this protocol was in effect.

He said the IAEA "was sending a serious and ominous message that failures (to comply) in the future will not be tolerated," a veiled threat that the Security Council remains an option.

Non-proliferation expert Gary Samore of London's International Institute for Strategic Studies told AFP that Iran's "main skeleton in the closet is the unexplained high enriched uranium particles" that agency inspectors have discovered at two sites in Iran.

Iran claims the particles came from contamination from equipment bought abroad but Samore said "the suspicion is that Iran may have acquired some enriched material on the black market and used the material as part of their centrifuge program" for enrichment of uranium to possible weapon-grade levels.

"If they failed to declare that, that would be a major safeguards violation," Samore said.

Joe Cirincione, a non-proliferation expert at the Carnegie Endowment for International Peace in Washington, said the United States was in fact in a good position in relation to Iran.

"They are getting more from Iran than they ever expected," he said about Iran's reporting fully since October about its nuclear program, allowing additional inspections and suspending the enrichment of uranium.

Cirincione said Iran needed several more years to acquire the technology to be capable of building the bomb and "they were caught much earlier than they wanted to be.

"They were stunned by the international reaction to the disclosures. No one is defending the Iranians," he said.

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