Under Kerry's proposal, Washington would lift its objections over Tehran's construction of nuclear power plants for civilian use if Iran agreed to abandon its efforts to keep nuclear fuel that can be used to make weapons, Edwards told The Washington Post.

If the Islamic republic rejected the terms of Kerry's offer, the United States would consider it a confirmation of Tehran's nuclear bomb ambitions, he said. The United States would call on European allies to impose heavy sanctions on Iran if it failed to comply.

"If we are engaging with Iranians in an effort to reach this great bargain, and in fact this is a bluff that they are trying to develop nuclear weapons capability, then we know that our European friends will stand with us," he told the Post.

Kerry's approach is a shift from President George W. Bush's policy. The two candidates will face off in the November 2 election.

The Republican president's administration opposes the activation of Iran's power plant in the Gulf port of Bushehr, which Tehran says is being built exclusively for civilian purposes.

Washington believes Iran, one of the world's largest oil suppliers, has no need for nuclear energy and accuses Tehran of hiding a nuclear weapons development program under the guise of a civilian atomic energy program.

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