(Adds quotes from UN Ambassador Susan Rice)

WASHINGTON, Feb 20 (Reuters) - Washington expressed alarm on Friday over new evidence that Iran understated how much uranium it had 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 it is purely for the peaceful generation of electricity. Enriched uranium can be used to make nuclear weapons.

Susan Rice, U.S. ambassador to the United Nations, said the report left no doubt about Iran's continued pursuit of its nuclear program, and Washington would use "enhanced pressures" and possibly "direct engagement" to convince it to change course.

"The United States views Iran acquiring an illicit nuclear capacity as a grave threat to ourselves, to the region and indeed to Israel," Rice told National Public Radio in an interview airing on Friday and Monday.

"Our aim is to combine enhanced pressures, and indeed the potential for direct engagement to try to prevent Iran from taking its program to fruition," she said. (Reporting by Ross Colvin; editing by Doina Chiacu; additional reporting by Andrea Shalal-Esa)

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