Iran's supreme leader Ayatollah Ali Khamenei said on Wednesday that any talks with arch-foe the United States would take place only if Washington drops "sanctions and threats" against Tehran.

In a speech on state television, the all-powerful Khamenei also blamed Washington for Iran's defiant move to enrich uranium to 20 percent, the most controversial aspect of its nuclear programme.

"The respected president (Mahmoud Ahmadinejad) and others have said that we are ready for negotiation. It is right. But not with America," Khamenei told an audience of senior Iranian officials, including Ahmadinejad, in the speech.

"The reason is that America does not enter the field honestly as a normal negotiator. They should drop the face of a superpower, they should drop threats, they should drop sanctions and they should not set a goal for negotiation. Then we are ready."

Washington, with whom Tehran has had no diplomatic ties for more than three decades, led world powers on June 9 in imposing new UN sanctions against Iran for continuing to enrich uranium.

Western and European nations have a dual-track policy of imposing sanctions and calling for talks to bring Tehran to the negotiating table to discuss its nuclear programme.

They suspect the programme masks a weapons drive, a charge Iran strongly denies.

Khamenei, the Islamic republic's spiritual guide who has the final say on all national issues, said that Iran has always "rejected" US offers for talks "because negotiations under threat are not negotiations."

"On one hand they threaten us and impose sanctions and show an iron hand, and on the other hand they want us at the negotiating table. We do not consider this as negotiations," he said.

"Experience has shown that when they cannot answer logic, they bully... we will not budge under pressures and we will respond to these pressures in our own way."

Earlier this month, Ahmadinejad urged the United States to join talks on a nuclear fuel swap deal for Iran.

He also offered to hold talks with US President Barack Obama on "global problems" at the UN General Assembly in September, although Washington rebuffed his proposal.

Later US Secretary of State Hillary Clinton said Washington remains "open to engagement" with Tehran.

"But they do know what they have to do. They have to reassure the international community by words and actions as to what their nuclear programme is intended for," she told The New York Times.

But she added that Washington would pursue the sanctions route "regardless of any issue of timing, because we think it's got the best potential for changing Iranian behaviour."

Khamenei on Wednesday blamed Washington for forcing Tehran to start refining uranium to 20 percent.

"In the past we purchased the 20 percent fuel, but as soon as America realised Iran needs 20 percent (enriched uranium), it made an issue of it. This was a major mistake by America and the West," he said.

He said Iran never wanted to enrich uranium to 20 percent.

"For us 3.5 percent was enough, but they forced our hand and made us realise that we need to do this and we did," Khamenei said.

He said the United States and others that produce this fuel "are not reliable and they will try to pressure you when they realise you need the fuel."

"We have found a way to produce the fuel and we will continue on this path."

In February, Ahmadinejad ordered Iranian atomic chief Ali Akbar Salehi to step up uranium refinement to 20 percent after a deal drafted by the UN atomic watchdog to supply the material to Tehran hit deadlock.

Iran says it needs 20 percent enriched uranium to power a research reactor in Tehran.

Western and European nations led by Washington strongly oppose Tehran's move to enrich uranium to this level, as they suspect the enrichment programme masks a weapons drive.

Experts say that by enriching uranium to 20 percent purity, Iran has theoretically come closer to the 90 percent purity needed for an atomic bomb.