"I am not in the business of telling Congress what to do," Ashton said in response to a question at a Washington think tank about whether the US Congress or others should impose additional sanctions on Iran.

"I would like to get to Geneva with the best possible atmosphere to really have these negotiations," she said, referring to October 15-16 talks between Iran and six major powers: Britain, China, France, Germany, Russia and the United States.

"And that means, in all sorts of ways, we need to show willingness and good faith to sit down and talk and expect the same in return," she added in an appearance at the Woodrow Wilson Center think tank.

Political directors from the six major powers - known as the P5+1 because they include the five UN Security Council permanent members and Germany - are to meet Iranian officials in Geneva to discuss Iran's nuclear program.

Tehran maintains that as a committed signatory to the Non-Proliferation Treaty (NPT) and a member of the International Atomic Energy Agency (IAEA), it has the right to use nuclear technology for peaceful purposes.

During the course of numerous inspections of Iran’s nuclear facilities by the IAEA, no evidence has ever been found showing that the Iranian nuclear energy program has been diverted toward non-civilian purposes.

"It may be, at the end of those two days that we don't make progress. But it may be ... that we do," Ashton said, saying her general approach to a negotiation is to keep pressure on.

"Pressure is there for a reason: it's to bring people to the talks in order to try and make progress," she said.

"In any thinking about that, those who are making the law here or those in control of the negotiations from the US end ... (US Secretary of State John) Kerry and his team will have to think about how to make sure that it's the best possible atmosphere."

NTJ/BA