The Iranian claims of major advances in its nuclear program came only a day after the Obama administration said it was ready to reverse previous U.S. policy and directly engage Iran over the issue.
But in a talk with reporters, Clinton declined to call the Iranian statements a rebuff to the U.S. overture and also expressed some skepticism that Tehran has actually made enrichment gains.
Iranian President Mahmoud Ahmadinejad Thursday opened a plant he said was capable of producing uranium fuel in industrial quantities and said Iran is testing two new high-capacity enrichment centrifuges.
An Iranian nuclear official announced separately that Tehran is running 7,000 enrichment centrifuges at its Natanz enrichment complex, more than 1,000 more than previously reported.
Clinton, speaking after U.S.-Australian security talks that included the Iranian nuclear issue, said U.S. officials do not attribute any particular meaning to the latest Iranian claims.
"We don't know what to believe about the Iranian program. We've heard many different assessments and claims over a number of years. One of the reasons we are participating in the P-Five-Plus-One is to enforce the international obligations that Iran should be meeting to insure that the IAEA is the source of credible information, because there is a great gap between what the IAEA observed about six or seven weeks ago, and what the Iranians are now claiming," she said.
In a mid-February report, the International Atomic Energy Agency said Iran had about four thousand centrifuges in operation at the Natanz plant and another 1,500 being tested.
Iran contends its enrichment operation is part of peaceful nuclear program while the United States and European allies believe it is at least partially weapons related.
The P5+1, the five permanent U.N. Security Council member countries and Germany, have offered Iran a range of economic and political incentives to end enrichment and return to negotiations over the program.
The Bush administration had refused to take a direct role in the talks unless Iran first suspended the enrichment drive.
But the State Department said Wednesday the United States is dropping that pre-condition and would sit at the table at the next P5+1 meeting with the Iranians, being arranged by European Union chief diplomat Javier Solana.
Clinton said Iran would benefit if it cooperated with the international community on the nuclear issue and abided by obligations of transparency in its program to which the United States believes Iran is bound.