U.S. Secretary of State Hillary Clinton on Tuesday denounced Iran's decision to begin enrichment operations at its new Qom facility, saying such an act runs counter to its obligations made under multiple UN Security Council and International Atomic Energy Agency Board of Governors resolutions.

"This step once again demonstrates the Iranian regime's blatant disregard for its responsibilities and that the country's growing isolation is self-inflicted," the top U.S. envoy said in a written statement.

Iran confirmed on Monday that it has begun uranium enrichment at its Fordo nuclear facility near the city of Qom, which is used for producing up to 20 percent enriched uranium.

"There is no plausible justification for this production. Such enrichment brings Iran a significant step closer to having the capability to produce weapons-grade highly enriched uranium," Clinton said.

She dismissed as "false" Iran's claims that its decision was necessary to produce fuel for the Tehran Research Reactor, saying the U.S., Britain, France, China, Russia plus Germany, the so- called P5+1, has offered alternatives for providing fuel for the reactor despite Iran's longstanding refusal to meet its international obligations.

She urged Iran to "immediately" cease uranium enrichment and return to negotiations with the P5+1 with readiness to engage " seriously" on its nuclear program.

"We reaffirm that our overall goal remains a comprehensive, negotiated solution that restores confidence in the exclusively peaceful nature of Iran's nuclear program while respecting Iran's right to the peaceful use of nuclear energy consistent with its obligations under the Nonproliferation Treaty," she said.

The U.S. and its European allies are mulling over expanded sanctions against Iran to target its oil exports, prompting the Islamic republic to threaten in late December to close the Strait of Hormuz, one of the world's most critical oil route.

The P5+1 last had talks with Iran in Istanbul, Turkey in January 2011, but made no progress.