Washington expressed alarm on Friday over new evidence that Iran understated how much uranium it enriched, calling Tehran's nuclear program an "urgent problem" the international community must address.

A new report by the International Atomic Energy Agency this week said Iran had built up a stockpile of nuclear fuel. Diplomats said Iran had underreported by a third how much uranium it had enriched but that the IAEA believed the discrepancy was a technical mistake rather than subterfuge.

"The report represents another lost opportunity for Iran as it continues to renege on its international obligations," said White House spokesman Robert Gibbs, echoing earlier comments by the U.S. State Department.

"It does underscore the urgency which the international community must work together to address these enrichment activities. This is an urgent problem that needs to be addressed and we cannot delay addressing," Gibbs said.

The United States suspects Iran of trying to use its nuclear program to build an atomic bomb, but Tehran insists that is purely for the peaceful generation of electricity. Enriched uranium can be used to make nuclear weapons.