"The International Atomic Energy Agency plans to go back to Parchin," the diplomat close to the Vienna-base IAEA told AFP, saying Iran had not yet granted permission for a second visit.

The diplomat, who asked not to be named, said IAEA inspectors wanted to go to another part of the sprawling Parchin complex, which they had first visited last Thursday after months of asking Iran for access.

But the diplomat said this was a routine part of a "process" and that "there is nothing like a fire."

"There are a number of things the IAEA wants to clarify according to its plans," the diplomat said.

Washington has voiced concern the Iranians may be testing high-explosive charges with an inert core of depleted uranium at Parchin, 30 kilometresmiles) southeast of Tehran, as a sort of dry test for how a bomb with fissile material would work.

Tehran has strongly denied carrying out any nuclear-related work at the site, which has buildings housing testing equipment, flanked by bunkers where explosions are carried out.

The IAEA has been investigating Iran for two years on US charges that the Islamic Republic is hiding nuclear weapons development.

IAEA chief Mohamed ElBaradei has said there is no proof that Iran is hiding weapons work but that "the jury is still out."

Allowing the Parchin inspection is a confidence-building measure by Iran which comes as it resumed talks last week in Brussels with the European Union on a trade accord, 18 months after negotiations were suspended due to concerns about Tehran's nuclear plans.

The negotiations on a trade and cooperation agreement were restarted after Iran agreed to suspend uranium enrichment -- the crucial part of the nuclear fuel cycle which can also make material for atomic bombs -- in an accord thrashed out following intense pressure notably from the United States.

The New Yorker magazine reported Monday that US commandos have been operating inside Iran since mid-2004 selecting suspected weapons sites for possible air strikes.

New Yorker journalist Seymour Hersh said US hawks are convinced European negotiations with Iran will fail, and when they do, the United States will act -- possibly by mid-year.

ElBaradei announced earlier this month that Iran had finally given the green light for his inspectors to probe Parchin after seeking access to the site since July.

The inspectors are taking environmental samples to check for radiation. Results from the highly sensitive sampling, which can detect minuscule amounts of radioactive particles even if a site has been cleaned, are available after about a month of laboratory analysis.

Iran had warned that it would not tolerate "spying" at the Parchin facility and would keep inspectors from entering buildings. But diplomats said UN inspectors had entered buildings last Thursday and expected to again in a second visit.

US non-proliferation expert David Albright, of the Washington-based Institute for Science and International Security, told AFP that the IAEA may find no more than anti-tank work using depleted uranium rockets.

But he said that even if the inspection did not find much it could still "create a precedent" for better access and "deter Iran from using the Parchin site since the presence of inspectors would remove that facility from the list of places where Iran can work on nuclear weapons."

Iran said Sunday it was confident that UN inspectors would disprove US allegations that it is conducting secret nuclear weapons work, and said its negotiations with the Europeans were "on a good track".

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