Any US attack on Iran would be a "nightmare," said Fischer during a discussion at Vienna's Institute for Human Sciences.

He described as "shocking" comments on the issue made by Rudolph Giuliani, who is one of the Republican candidates for the 2008 presidential election.

Giuliani said last month: "If I am President of the United States, I guarantee you, we will never find out what they will do if they get nuclear weapons, because they are not going to get a nuclear weapon."

"The military option is not off the table."

Fischer warned the United States not to start again a war that could not be won. An attack on Iran would noly boost Iranian President Mahmoud Ahmadinejad's chances in or re-elections next year.

The United States is spearheading international efforts to thwart Iran's atomic work but Iran denies Western charges that it is trying to build atomic weapons under the guise of its civilian nuclear programme.

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