Kerry met Egyptian Foreign Minister Sameh Shoukry in the morning and was to later meet President Abdel Fattah al-Sisi before flying to Qatar to meet Gulf Arab counterparts.
Egypt, and other countries in the region such as Saudi Arabia, are suspicious of Iran, which they view as bent on destabilising their countries.
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"There can be absolutely no question that if the Vienna plan is fully implemented, it will make Egypt and all the countries of this region safer than they otherwise would be or were," Kerry told a Cairo press conference after meeting Shoukry.
Read: Good tidings: Iran deal brightens IP gas pipeline prospects
"The United States and Egypt recognise that Iran is engaged in destabilising activities in the region and that is why it is so important to ensure that Iran's nuclear programme remains wholly peaceful," he said.
"If Iran is destabilising, it is far, far better to have an Iran that doesnt have a nuclear weapon than one that does."
Kerry's trip, which ends on August 8, will not include Israel, one of Washington's closest allies, which has been a fierce critic of the July 14 nuclear deal between the world powers and Iran.
In Doha, Kerry will meet his counterparts from the six Gulf Cooperation Council (GCC) member states
He will seek to allay their fears about Shiite Iran, following the nuclear deal signed in Vienna.
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