"The negotiations will go on if there is progress at (Friday's) meeting," said Ali Agha-Mohammadi, a spokesman for supreme national security council which is charge of the nuclear dossier.

"But if the Iranian delegation does not see any progress then the process will change radically," he said on state radio ahead of the London meeting.

"(In that case) there is no question that we would reconsider the negotiations process because that would be a violation of the Paris accord," he said, referring to a deal in late 2004 that paved the way for the talks.

The European Union, represented by Germany, Britain and France, is demanding that Tehran abandon nuclear fuel work to guarantee it will not make atomic weapons.

Tehran froze enrichment last November as a confidence-building measure to start the talks with the EU, which is offering trade, security and technology rewards if it makes the suspension permanent.

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