Iranian President Mahmoud Ahmadinejad said Iran has "no secrecy." (Vahid Salemi/Associated Press)

UN nuclear inspectors got a tour of a previously undisclosed uranium enrichment facility in Iran that raised concerns in the West about the extent and intent of its nuclear program.

State media reported that four members of the International Atomic Energy Agency visited the underground nuclear facility, which is being built into the side of a mountain near Qum, about 150 kilometres southwest of Tehran.

The United States, Britain, France, Germany, Russia and China demanded unfettered access to the facility after Iran disclosed its existence last month in a letter to IAEA.

At the time, Iranian President Mahmoud Ahmadinejad told the UN that the country did not keep its second enrichment facility secret and that Iran was working within the guidelines of the UN nuclear watchdog.

But he dodged a question about whether Iran had enough enriched uranium to make a nuclear weapon. Ahmadinejad claimed the rules required that the agency be informed of any new enrichment facility six months before it became operational.

The new facility wouldn't be working for 18 months, he said.

But the IAEA said Iran was obliged to notify the agency when it begins designing such facilities. Uranium enrichment is a process that can be used to make fuel or nuclear weapons.

No results from Sunday's inspection are expected until the team leaves the country. The four members from IAEA are expected to spend three days in Iran.

They are expected to compare Iran's engineering plans with the actual layout of the plant, interview employees and take environmental samples to check for the presence of nuclear materials.

The small-scale site is meant to house no more than 3,000 centrifuges — much less than the estimated 8,000 machines at Iran's other uranium conversion facility, Natanz, south of Tehran.

A recent satellite image provided by DigitalGlobe and GeoEye shows a well-fortified facility built into a mountain about 32 kilometres northeast of Qum, with ventilation shafts and a nearby surface-to-air missile site, according to defence consultancy IHS Jane's, which did the analysis of the imagery. The image was taken in September.

With files from The Associated Press