A team of International Atomic Energy Agency (IAEA) inspectors arrived in Tehran on Saturday night to visit a new nuclear site south of the Iranian capital that has created new suspicions about Iran's nuclear programme.
Flying from Vienna, where the IAEA is seated, the four experts were set to visit the unfinished uranium enrichment site near the village of Fordo for the first time since Iran informed the IAEA about it in late September, years after starting construction.
The inspection is one of the confidence-building steps agreed between Iran's and the five permanent UN Security Council members plus Germany in Geneva Oct 1.
The underground Fordo site has been under construction for the past few years and is located 47 km south of the religious city of Qom in a compound protected by the Revolutionary Guards.
Diplomats and experts say there is likely no technical equipment installed yet in the facility that is Iran's second nuclear enrichment plant besides the one already operating in Natanz.
The three men and one woman on the IAEA team headed by Herman Nackaerts are set to compare technical plans of Fordo with the actual layout of the plant, check for the presence of nuclear material and discuss the site's role within Iran's nuclear programme, according to ElBaradei and officials familiar with the IAEA.
But it was less the site itself than the late timing of Iran's declaration to the IAEA that has Western countries worried.
Tehran argues it has the right to not implement an IAEA rule under which countries have to inform about new nuclear facilities as soon as they decide to build them. Instead, Iran only accepts an older rule that called for only six months advance notice.
But ElBaradei has said the Islamic state is "on the wrong side of the law" with its stance.
The other question that experts and Western countries would like to see answered is why Iran needs a second enrichment plant that is too small to create enough fuel for Iran's power plant being constructed in Bushehr.
Enrichment technology can be used for making nuclear fuel for power reactors, but also for nuclear weapons. Tehran denies it has any plan to build atomic bombs.
Iran's IAEA ambassador Ali Asghar Soltanieh has argued that Fordo was a back-up plant in case the Natanz site was attacked by Israel.
"Therefore we had to have a contingency plan in order to have a sustained enrichment programme," he told the BBC recently.
Israel's policy is to keep open the option of attacking Iran over its contentious nuclear programme. Israel has launched attacks on Iraq and Syria in the past, in order to destroy nuclear facilities there.
The IAEA inspection that is to last until about next Wednesday comes just a few days before the permanent UN Security Council members Britain, China, France, Russia and the US, plus Germany, are set to meet Iranian representatives for another round of nuclear talks in Geneva in the second half of the coming week.
Although it is unlikely that the IAEA will be able to provide an analysis of its inspection by then, diplomats have told DPA that the outcome of the visit would influence the talks.