VIENNA - Iran's announcement that it plans to triple its production capacity of higher-grade uranium is a "brazen" example of Tehran's defiance of international obligations, a senior U.S. official said on Thursday.

Iran also said on Wednesday it planned to shift the production of the material to an underground bunker this year.

Western powers are concerned about the higher-grade enrichment because it takes Iran a step closer to producing potential atomic bomb-grade fuel. Tehran denies such aims and says its nuclear program is for peaceful uses only.

Speaking at an International Atomic Energy Agency (IAEA) board meeting in Vienna, Washington's envoy Glyn Davies said the plan was Iran's "most recent brazen example of its deepening non-compliance."

Iran only disclosed the existence of Fordow, a mountain bunker near the clerical city of Qom, in September 2009, after Western intelligence had detected it and said it was evidence of covert nuclear work.

"Apart from what appears now to be a clear intent to produce more 20 percent enriched uranium than Iran needs to make fuel for its one and only research reactor, it also represents yet another chapter in the changing Iranian narrative regarding why this underground facility was built," Davies said.

The IAEA reiterated on Thursday that Iran had not notified the agency of its enrichment plan.