"Over time we believe that the strategy will build, the capacity will build, Daesh will become more isolated," he said on Monday (AEST), referring to the jihadist IS by its acronym in Arabic.
"But ultimately it is Iraqis who will have to take back Iraq. It is Iraqis in Anbar who will have to fight for Anbar."
Kerry also called the IS offensive on the Syrian border town of Kobane a "tragedy", despite weeks of US-led air strikes.
But he stressed that the Syrian-Kurdish city, under siege from Islamic militants, was "just one community" and it "does not define the strategy of the coalition with respect to Daesh".
UN officials have warned of a possible massacre in Kobane, if the militants capture the city which they have been attacking for the past three weeks.
The US has repeatedly highlighted its concerns for Kobane, but stressed that saving it from falling is not Washington's top priority in the fight against IS.
"We have said from day one that it is going take a period of time to bring the coalition properly to the table," Kerry told reporters after a day of talks in Cairo devoted to raising money to rebuild Gaza.
It will take time to "rebuild some of the morale and the capacity of the Iraqi army" which simply evaporated in face of a lightning June offensive by the Islamic militants in which they seized a swathe of territory.
"There will be ups and there will be downs over the next days, as there are in any kind of conflict," Kerry said.
"But we are confident about our ability to pull this strategy together, given the fact that every country in the region is opposed to Daesh."
The top US diplomat also said that more than 60 countries had now pledged to help the fight against IS.
During his day in Cairo, Kerry also met with his Jordanian and Qatari counterparts for talks which focused on the US-led coalition which both nations have joined.
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