However, IAEA Spokeswoman Gill Tudor said that all nuclear materials and operations in the Fordo facility are “under the Agency's containment and surveillance."

Iranian officials earlier said the Fordo plant, deep inside the mountains near the central Iranian city of Qom, was build to produce 20-percent uranium needed for a research reactor in Tehran, which produces medical isotopes to treat cancer patients.

The head of the Atomic Energy Organization of Iran (AEOI) Fereydoon Abbasi said last August that Iranian authorities planned to transfer all enrichment facilities from a plant in Natanz to Fordo, citing insufficient security.

He also said Iran had no plans to enrich uranium to higher than 20 percent.

The news is likely to further increase tensions between Iran and the West over the country’s clandestine nuclear research. Western nations suspect Iran of pursuing a secret weapons program while Tehran says it needs nuclear research for peaceful energy purposes. Negotiations have been stalled for 12 months.

The IAEA statement has already triggered harsh statements from diplomats in the United States and Europe.

"This is a further escalation of their ongoing violations with regard to their nuclear obligations," State Department spokeswoman Victoria Nuland told reporters.

"The fact that the IAEA has made clear that they are enriching to a level that is inappropriate at Fordo is obviously a problem," she went on.

UK Foreign Secretary William Hague described the move as "a provocative act that undermines claims that the program is civilian in nature."

The German foreign ministry expressed hope that the European Union will agree on a fresh round of sanctions against Iran during a meeting on January 30, because the start of operations at Fordo was “a step of further escalation.”

“With it, the international community’s concern that the Iranian nuclear program is serving military purposes is growing,” the ministry said in a statement. “So long as Iran does not move, there is no alternative to tough sanctions,” the statement said.