Iran had promised "frequent inspector access" to the site in Natanz, the UN watchdog International Atomic Energy Agency (IAEA) reported in February.

The highly sensitive inspections, and talks over how they are to take place, came as Iranian President Mahmoud Ahmadinejad was expected to plead Iran's case this week before the UN Security Council, which is considering tightening sanctions on the Islamic republic over fears that it seeks nuclear weapons.

A centre of concern is the uranium enrichment facility at Natanz, in central Iran, where the Iranians are already operating above-ground a pilot plant carrying out research levels of enrichment, the process which makes what can be fuel for civilian nuclear reactors but also the explosive core of atom bombs.

A diplomat said Iran had last Saturday refused to let IAEA inspectors into the underground hall at Natanz where the Iranians have set up hundreds of centrifuges in what is to be a 3,000-centrifuge facility for enriching uranium.

Centrifuges are the machines used to refine uranium for the U-235 isotope that is valuable for fuel or weapons.

Such a facility could make enough highly enriched uranium for an atom bomb in about 10 months, according to the IISS think-tank in London.

Other diplomats, all requesting anonymity due to the extreme delicacy of the issue, said IAEA inspectors are set to return this week, possibly Tuesday, to the plant and that delays in inspections were normal and could just be a matter of schedule changes or working out legal issues.

Blocking access definitively to Natanz would be a violation of Iran's obligations under the nuclear Non-Proliferation Treaty (NPT).

Iranian officials in Vienna were not immediately reachable for comment.

IAEA chief Mohamed ElBaradei had reported on February 22 that the agency wanted to put cameras in the centrifuge hall in accordance with "safeguards measures that needed to be implemented . . . prior to the introduction of nuclear material into the facility."

The report said that Iran was spinning centrifuges empty, without uranium feedstock gas, in two 164-centrifuge production lines, and was finishing installing two other similar cascades, also totalling 328 centrifuges.

ElBaradei said the IAEA had "agreed to interim verification arrangements" at Natanz's underground site "involving frequent inspector access but not remote monitoring."

"Iran was informed that these arrangements (which are now in place) would be valid only for as long as the number of machines installed ... did not exceed 500, and that, once that number was exceeded, all required safeguards measures would need to be implemented," ElBaradei said.

The first diplomat said Iran did not want the IAEA to see "that it now has more than 500 centrifuges functioning underground" and that was the reason for the delay.

Tehran is defying the UN Security Council's calls for it to suspend uranium enrichment. Iran insists its nuclear program is a peaceful effort to generate electricity.

Iran warned on Monday that it would make a "proportionate" response to any new UN sanctions.

Diplomats in Vienna speculated that cutting off access to Natanz might be part of this response.

The 15-member Security Council is due to meet Wednesday to review a draft sanctions resolution against Iran agreed last week by the body's five permanent members -- Britain, China, France, Russia and the United States -- plus Germany.

New sanctions would include barring Iran from exporting arms and buying weapons such as missiles.

Iran would have 60 days to comply with repeated UN demands or face "further appropriate measures" (economic sanctions but no military action) under Article 41 of the UN Charter.

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