The Obama administration has condemned Iran for enriching uranium to a level that can be upgraded quicker for use in a nuclear weapon than the nation's main stockpile.
Secretary of State Hillary Rodham Clinton says Tehran is breaking its international obligations and demonstrating a "blatant disregard for its responsibilities."
Clinton's statement said enrichment at the Fordo site was especially troubling because Iran previously sought to hide the facility from U.N. monitors. She said "there is no plausible justification" to enrich to 20 percent and that the step brings Iran closer to nuclear weapons capacity.
Iran insists that its program is designed for energy and research purposes.
But Clinton said Iran must halt its enrichment and urged it to return to negotiations.
Several other countries have condemned Iran's move.