Iran on Monday confirmed it has started uranium enrichment at a second facility where the material can be upgraded quickly for potential use in a nuclear bomb.

Iran's envoy to the International Atomic Energy Agency, Ali Asghar Soltanieh, said Tehran is refining uranium at the newly-launched Fordo complex and an older facility in the city of Natanz. He told Iran's Arabic language al Alam TV network the work was taking place under the supervision of the United Nations atomic agency.

Diplomats with ties to the IAEA confirmed that Iranian centrifuges have begun refining uranium to a purity of 20 percent at Fordo, an underground complex near the Shi'ite holy city of Qom.

The United States and France condemned the move.

U.S. State Department spokeswoman Victoria Nuland questioned Iran's motives. She said that when uranium enrichment is raised to 20 percent, “it generally tends to indicate that you are enriching to a level that takes you to a different kind of nuclear program.”

France called the move “this new provocation.” A Foreign Ministry statement said the move “leaves us with no other choice but to reinforce international sanctions and to adopt, with our European partners and all willing countries, measures of an unprecedented scale and severity.”

The U.N. Security Council has imposed four sets of sanctions on Iran for refusing to stop enrichment work, which has civilian and military uses. Western powers accuse Iran of trying to develop an atomic weapons program. Tehran says its nuclear ambitions are peaceful.

The United States and the European Union have been tightening their own sanctions on Iran to pressure it into suspending enrichment. Iranian Supreme Leader Ayatollah Khamenei said Monday the sanctions will not achieve their goals and vowed to resist the pressure.

The Fordo complex is located beneath a mountain and is better protected from potential airstrikes by nations opposed to the Iranian nuclear program. Iran said previously it was preparing to move its highest-grade enrichment work to Fordo from its an above-ground plant in Natanz.

Most of the work at the Natanz facility has involved refining uranium to a relatively low purity of 3.5 percent. Enrichment to the 20 percent level at the Fordo complex could reduce the time needed for Iran to further refine the material to the 90 percent purity required for nuclear weapons.

Iran says its nuclear program is designed only to generate electricity and material for medical research.

The United States and its ally Israel have not ruled out military action to stop the program. Israel sees a nuclear-armed Iran as a threat to its existence.