WASHINGTON - Three weeks after President Obama hailed a landmark deal to suspend most of Iran's nuclear program for the next six months, the mood among U.S. officials about the next round of negotiations has shifted from elated to somber.

"I wouldn't say [chances of success are] more than 50-50," Obama said last week. U.S. officials are "very skeptical" that Iran will accept Western demands, said his lead negotiator, Wendy R. Sherman.

The shift, officials say, is the result of a growing recognition of the compromises each side must make to resolve the decade-old impasse over Western suspicion that Iran will someday try to build nuclear weapons and the Iranian demand that the sanctions crippling its economy be lifted.

Technical talks in Vienna aimed at implementing the initial deal stopped last Thursday.

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