"We have no experience with the new American government and the new American president. We will observe them and we will judge," said Ayatollah Khamenei in a speech carried live by Iranian television.

"If you change your attitude, we will change our attitude."

He said Iran was yet to see such a change.

"We cannot see any change," he said. "What is the change in your policy?

"Did you remove the sanctions? Did you stop supporting the Zionist regime? Tell us what you have changed. Change only in words is not enough."

Ayatollah Khamenei's comments echoed those of an adviser to Iranian President Mahmoud Ahmadinejad, who said on Friday that Washington must reverse its support for Israel and end sanctions against Iran.

Mr Obama's offer came in a direct video address to mark the Iranian New Year.

In the message, Mr Obama said he was seeking engagement with Iran that was "honest and based on mutual respect".

"My administration is now committed to diplomacy that addresses the full range of issues before us," he said.

The address was distributed to news outlets in Iran with subtitles in Farsi, and posted on the White House website.

Mr Obama's approach - prefigured in his inauguration speech - was seen as a dramatic departure from the policies of the Bush administration, which described Iran as part of the "axis of evil".