France's reiteration of its tough line came after the United States was forced to deny reports that it was ready to soften its own stance and open negotiations without Tehran first halting uranium enrichment.

A spokesman for the French foreign ministry said that France stood by the proposition made to Tehran in 2007 by the six-nation Iran contact group -- Britain, China, France, Germany, Russia, the United States.

Under this initiative, Tehran would halt all nuclear activity beyond its civilian site in Bushehr, including all uranium enrichment, while the contact group would in exchange promise not to seek new UN economic sanctions.

During the limited period of this so-called "double freeze", Tehran and the contact group could begin substantial negotiations on the future of Iran's nuclear programme, spokesman Romain Nadal said.

Like the United States, Nadal said, France was open to a "dialogue" with Iran at any time to improve relations, but said formal talks on the nuclear programme could only go ahead under the conditions of the 2007 offer.

On Tuesday the US press reported that President Barack Obama was preparing to change tack and open talks with Tehran without a prior enrichment freeze. The White House denied the reports.

Western powers fear that Tehran's nuclear programme is aimed at building an atomic bomb, but Iran insists that it merely seeks to produce peaceful civilian energy.

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