VIENNA: Iran has further stoked Western fears about the purpose of its nuclear activities by starting to enrich fissile material in a new site deep inside a virtually impregnable mountain bunker.

The Fordo site near the holy city of Qom, 150 kilometres (90 miles) southwest of Tehran, has begun enriching uranium to 20-percent purity, the UN atomic watchdog Agency, said late Monday.

US State Department spokeswoman Victoria Nuland said that Iran's enriching of uranium to 20 percent at the site was "a further escalation of their ongoing violations with regard to their nuclear obligations."

"We call on Iran once again to suspend enrichment activities, cooperate fully with the IAEA and immediately comply with all Security Council and IAEA board of governors resolutions," Nuland told reporters in Washington.

Experts point out that the process of obtaining 20-percent enriched uranium represents the lion's share of the work needed to get the material purified to the level of 90 percent or above required for atomic weapons.

Iran, which says its nuclear activities are peaceful, says the material is for its Tehran research reactor to make isotopes for cancer treatments, but Western powers believe it has a much more sinister and destructive purpose.