The International Atomic Energy Agency -- the United Nations' atomic watchdog -- said that Iran has started enriching uranium at a fortified underground site in Fordow, near the city of Qom.

“The IAEA can confirm that Iran has started the production of uranium enriched up to 20 percent in U-235 using IR-1 centrifuges in the Fordow Fuel Enrichment Plant,” the agency said in a statement.

“All nuclear material in the facility remains under the agency’s containment and surveillance.”

The site can withstand multiple air-strikes, according to The Associated Press. The Fordow site is actually smaller than other nuclear facilities but it is said to be more efficient and is also hidden from surveillance.

Iran has continually stated that its nuclear program is only for peaceful purposes and the production of energy. Since issuing its report on Iran in November, the IAEA has maintained that within the past few years Iran worked towards creating a nuclear weapon, and the enrichment figures indicate that the program may have never stopped, as previously suspected.

“When you enrich to 20 percent there’s no possible use if you’re talking about a peaceful purpose,” U.S. State Department spokeswoman Victoria Nuland told reporters on Monday. “You’re enriching to a level that takes you to a different kind of nuclear program.”

“This is a further escalation of their ongoing violations with regard to their nuclear obligations,” Nuland noted Monday. “We call on Iran once again to suspend enrichment activities.”

The United States and the European Union are trying to use economic sanctions to persuade Iran to turn away from its nuclear ambitions, but so far the economic measures have only increased the tension between Iran and the rest of the world.

“The supreme authorities ... have insisted that if enemies block the export of our oil, we won’t allow a drop of oil to pass through the Strait of Hormuz. This is the strategy of the Islamic Republic in countering such threats,” Revolutionary Guard deputy commander Ali Ashraf Nouri told state news agencies

Iran has been enriching uranium for about five years and increased its efforts by 20 percent in February 2010. Additionally, the country produced its first nuclear fuel rods earlier this month.