"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 joint news conference with Shoukry of the landmark nuclear deal.
Egypt and other regional states such as Saudi Arabia are suspicious of Iran, which they see as bent on destabilising them.
Kerry said the US recognised 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".
"If Iran is destabilising, it is far, far better to have an Iran that doesn`t have a nuclear weapon than one that does," he said at the Cairo press conference.
On Monday, Kerry will meet his six counterparts from the Sunni-dominated Gulf Cooperation Council to try to allay their fears about Shiite Iran after the nuclear deal.
"This is an opportunity, really, for the secretary to do a deep dive with the GCC foreign ministers to try to respond to any remaining questions that they might have and hopefully to satisfy them and ensure that they`re supporting our effort going forward," a State Department official said.
The GCC comprises Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates.
Many Gulf Arab states have said they are concerned about Iran`s regional ambitions following the accord with the United States and Britain, China, France, Germany and Russia.
"New atmosphere"
Iranian President Hassan Rouhani meanwhile said in a live televised address Sunday that the July 14 nuclear agreement had created better prospects for faster solutions in Syria and Yemen, two of the Middle East`s worst conflict zones.
"The final solution in Yemen is political, in Syria the final solution is political," he said. "The agreement will create a new atmosphere. The climate will be easier."
On the sidelines of Monday`s GCC gathering in Doha, Kerry will hold a three-way meeting with Russian Foreign Minister Sergei Lavrov and his Saudi counterpart Adel bin Ahmed al-Jubeir.
"A key topic of discussion is expected to be the ongoing crisis in Syria," a senior State Department official said.
While in Cairo, Kerry held the first "strategic dialogue" with his Egyptian counterpart since 2009.
The United States has been working to patch up troubled ties with Egypt, long a key Middle East ally, as President Sisi battles an Islamic State insurgency in the Sinai Peninsula.
An Egyptian foreign ministry statement said both sides would keep cooperating closely "to improve their mutual security, to combat terrorism and extremism".
Ties between the US and Egypt frayed after then army chief Sisi overthrew Islamist president Mohamed Morsi in 2013.
More than 1,000 of Morsi`s supporters were killed in a sweeping crackdown, and militants have since killed hundreds of soldiers and policemen.
Most of the attacks have been by the Egyptian affiliate of the jihadist Islamic State group, which a US-led coalition is battling in Syria and Iraq.
Washington froze arms deliveries to Cairo following the crackdown on Morsi`s supporters, but resumed full aid in March and delivered a batch of F-16 jets last week.
"We have significantly increased military cooperation as seen from the delivery of the F-16s, other equipment and goods which are very essential in the fight against terrorism," Kerry said.
But the top US diplomat also spoke of the need for a "balance" between fighting militants and respecting human rights in Egypt.
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 nuclear deal.
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