"I think we have to approach this with a big grain of salt. President (Mahmoud) Ahmadinejad said one thing to the Iranian people on Saturday and another thing to an American journalist on Monday," she said.

"So I think that all of us need to consider this with a healthy dose of skepticism."

She was responding to an interview aired in part on the US television network NBC in which Ahmadinejad struck a relatively conciliatory tone.

If recent US behavior represented a genuinely new approach to Tehran, "we will be facing a new situation and the response by the Iranian people will be a positive one," Ahmadinejad said through an interpreter in a rare interview with a US broadcaster.

On Saturday, however, the president announced that Iran had boosted the number of uranium-enriching centrifuges to 6,000, in an expansion of its nuclear drive that defies international demands for a freeze.

The United States, in a break with past policy, took the unprecedented step of sending a top diplomat to meet Iran's chief negotiator at recent talks in Geneva over Tehran's disputed nuclear program.

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