US Secretary of State Hillary Clinton said on Tuesday Iran's decision to enrich uranium near the city of Qom was "especially troubling" and urged Tehran to return to serious talks with Western powers over its atomic program.
"This step once again demonstrates the Iranian regime's blatant disregard for its responsibilities and that the country's growing isolation is self-inflicted," Clinton said in a statement.
Clinton's strongly worded comments repeated US concerns over Iran's announcement that it had started enrichment at the underground Fordow bunker near Qom, which came amid rising tensions between Tehran and western powers.
Iran denies Western suspicions that its nuclear program has military goals, saying it is for purely peaceful purposes.
"The circumstances surrounding this latest action are especially troubling," Clinton said.
"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 rejected Iran's assertion that it needed to enrich uranium to produce fuel for a medical research reactor, saying Western powers had offered alternatives means of obtaining such fuel but their offers had been rejected by Tehran.
The United States imposed additional sanctions on Iran last month and the European Union is expected to agree on a ban on imports of Iranian crude oil later this month.
As sanctions squeeze, Iran has threatened to shut the Strait of Hormuz, the outlet for 40 percent of the world's traded oil.
At the same time, it has called for fresh nuclear talks with the permanent members of the UN Security Council and Germany, a group known as the "P5+1," which have been stalled for a year.
Calling on Iran to immediately halt uranium enrichment, Clinton also urged Tehran to return to talks with the P5+1 "prepared to engage seriously on its nuclear program."
"We reaffirm that our overall goal remains a comprehensive, negotiated solution," Clinton said.
Daryl Kimball, executive director of the nonprofit Arms Control Association, welcomed Clinton's fresh call for a negotiated solution.
"The United States and its 'P5+1' partners - China, France, Germany, Russia, and the United Kingdom - should continue to prepare for and more energetically pursue additional talks with Iran and continue to highlight constructive proposals they are prepared to discuss," Kimball said in an emailed comment.
"A near-term goal should be to test Iran's recent publicly stated offer to halt uranium enrichment to 20 percent levels if it could have access to fuel for its Tehran Research Reactor," he added.
"A stockpile of 20 percent would allow Iran to shorten its time frame to produce weapons, if it chose to do so," Kimball said. "We should not forgo any realistic opportunities to reduce that risk."