UN nuclear inspectors starting a two-day visit to Tehran on Monday sought to meet Iranian nuclear scientists and visit a key military facility as they try to gauge allegations that Iran is pushing toward making an atomic weapon.

The trip is the second in less than a month by the International Atomic Energy Agency team, reflecting growing concerns over alleged weapons experiments — something Iran has so far both denied and refused to discuss.

Herman Nackaerts, a senior UN nuclear official, said in Vienna before the team departed on Sunday that he hoped for progress in the talks but his careful choice of words suggested little expectation the meeting will be successful.

The trip came as Iran announced air defence wargames to practice protecting nuclear and other sensitive sites, the latest in a series of military manoeuvrs viewed as a message to the West that Iran is prepared both to defend itself against an armed strike and to retaliate.

The U.S. and Israel have not ruled out military action against Iran's nuclear program, which they say is geared toward making weapons. Iran denies the charges and insists the program is for peaceful purposes only, such as power generation.

Iran's state radio said Monday the IAEA inspectors hope to meet Iranian nuclear scientists and visit the Parchin military complex. The report said the IAEA had requested to visit Parchin, an Iranian military base and conventional weapons development facility outside of Tehran. The site has also been suspected of housing a secret underground facility used for Iran's nuclear program, a claim denied by Iranian authorities.

IAEA inspectors visited the site in 2005, but went to only one of four areas of potential interest within the grounds. At the time, the nuclear watchdog did not report any unusual activities but the Parchin site was prominently mentioned in the agency's report last year.

Purported nuclear blast 'vessel'

The report asserted that Iran constructed "a large explosives containment vessel" in which to conduct experiments on triggering a nuclear explosion, apparently 11 years ago, adding that it had satellite images "consistent with this information."

"Whatever the reasoning of the agency is, it proves the IAEA is not loyal to its previous commitments," the Iranian radio said. The tone of the commentary suggested the visit to the military complex would likely be denied.

The IAEA visit comes as Iran last week announced what it described as key advancements in its nuclear program, inserting the first domestically made fuel rod into a research reactor in Tehran and installing a new generation of Iranian-made centrifuges at the country's main uranium enrichment facility in the central town of Natanz.

Beyond concerns about the purported weapons work, Washington and its allies want Iran to halt uranium enrichment, which they believe could eventually lead to weapons-grade material and the production of nuclear weapons. Iran has been enriching uranium up to 20 per cent, while uranium enriched to more than 90 per cent can be used for a nuclear warhead.

The IAEA team wants to talk to key Iranian scientists suspected of working on an alleged weapons program. They also hope to break down opposition to their plans to inspect documents related to nuclear work and secure commitments from Iranian authorities to allow future visits.

The IAEA summarized its information last November in a 13-page document drawing on 1,000 pages of intelligence. It stated then for the first time that some of the alleged experiments can have no other purpose than developing nuclear weapons.

Iran has denied alleged weapons experiments for nearly four years, saying they are based on "fabricated documents" provided by a "few arrogant countries" — a phrase authorities in Iran often use to refer to the U.S. and its allies.