"Are we going to be able to close this final gap so that (Iran) can reenter the international community, sanctions can be slowly reduced and we have verifiable, lock tight assurances that they can't develop a nuclear weapon," Obama told CBS News in an interview broadcast Sunday.
"There's still a big gap. We may not be able to get there."
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His remarks came as America's top envoy, Secretary of State John Kerry and Iran's Foreign Minister Mohammad Javad Zarif started talks Sunday in Oman, the latest diplomatic effort ahead of a November 24 nuclear deal deadline.
Iran and the P5+1 powers -- Britain, China, France, Russia, the United States plus Germany -- are seeking an agreement on reining in Iran's nuclear program, in exchange for sanctions relief.