The United States wants the UN Security Council to approve a resolution within weeks, not months, laying the ground for new sanctions against Iran over its nuclear programme, the Pentagon said.

US Defense Secretary Robert Gates spoke to leaders in Turkey, Italy and France about the "urgent need" to move forward on sanctions as soon as possible, Pentagon press secretary Geoff Morrell told reporters.

Asked how quickly sanctions could be in place, Morrell said Gates, who visited Paris this week, believed it could happen "in weeks, not months."

In an interview with Fox News, a transcript of which was released on Tuesday, Gates said: "It is always a negotiating process and we're just at the beginning of it."

"I think it's going to take some period of time; I would say weeks, not months, to see if we can't get another UN Security Council resolution," he added.

Asked what would happen if China or any other country opposed a resolution, Gates said: "All I can say is we have been successful in getting several security council resolutions so I'm optimistic we'll be successful again."

Morrell said Gates "clearly thinks time is of the essence" to approve the resolution in order to provide a "legal platform" for the European Union and others to "take action on their own."

Iran began work on Tuesday to produce higher-grade nuclear fuel for a Tehran research reactor, an official said. It aims to produce uranium enriched to a level of 20 percent for a reactor making medical isotopes.

This followed a failure to agree terms for a proposed nuclear fuel swap with major powers, under which it would send most of its low-enriched uranium abroad.

• Iran begins controversial new nuclear process