"I personally believe we ought to give some final vigorous push to diplomacy to try to deal with this," Clinton told the World Economic Forum, lending implicit support to European Union efforts at engaging Tehran on the issue.

"I'm not entirely convinced that what our British and German and French and other friends are trying to do won't work," he said, in reference to delicate negotiations by the three major EU powers over Iran's nuclear program.

Iran, which Washington accuses of trying to covertly develop nuclear arms, has suspended uranium enrichment as a confidence-building measure, but the EU now wants the Islamic republic to definitively abandon enrichment as well as any activities for making plutonium.

Authorities in Iran have denied that talks were at an impasse, the Iranian press reported Thursday.

Clinton said he did not think Tehran would use any nuclear weapons that it developed because it would trigger a destructive retaliatory strike, but that radioactive material could fall into the hands of terror networks.

"If Iran had a nuclear weapon, the main thing it would do would be to cast a pall over the Middle East, but it would take a long time before they used it because they'd be toast if they used it," he said.

The real concern, he added, was that fissile material could be smuggled out of Iran and given to a terrorist group.

"We now know this, you can get on the Internet and see this, but if you have basically a cookies' worth of fissile material and you put it into a traditional bomb, you can amplify the destructive power by a hundred fold, or more.

"The reason you don't want Iran to have an active nuclear program is that given the present state of play you will never know if the materials are secure or being transported to terrorist networks."

Clinton said he was not sure the United States could attack sites in Iran, as Israel did in Iraq when Baghdad sought to build a nuclear reactor, because it was a more formidable foe and there would be heavy civilian casualties.

But, he insisted, "there ought to be some sort of mega deal" available to enable Iran to accept the West's demands as its religious leadership was not beyond reach.

"The religious council in Iran has not entirely shut down their democracy, they haven't totally invalidated everything they've tried to do," he said.

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