Kerry told the first high-level talks in Brussels of a coalition of around 60 nations that strikes in Iraq and Syria were having a "significant" impact and vowed that they would stay the course.
The Pentagon said Iranian F-4 Phantom jets hit IS fighters in eastern Iraq in recent days but denied any military coordination with long-term adversary Iran. Tehran refused to confirm or deny any strikes.
Kerry, opening the talks at NATO headquarters, told foreign ministers taking part that the coalition of western and Arab states "will engage in this campaign for as long as it takes to prevail".
"Our commitment will most likely be measured in years," he said.
But he added that "our efforts are already having a significant impact".
Kerry made no comment on Iran, while Iraqi Prime Minister Haider al-Abadi, asked about the Iranian action, said he had "no knowledge of any Iranian air strikes."
Saudi Arabia, the United Arab Emirates, Jordan and Bahrain are taking part in the air strikes in Syria. Australia, Belgium, Britain, Canada, Denmark, France and the Netherlands are participating in Iraq.
Dozens more countries are offering other support including information and intelligence sharing.
The United States has carried out the vast majority of the air strikes against the jihadists.
It says there are around 30,000 IS fighters in both Syria and Iraq, where the group is accused of atrocities including rape, crucifixion and the beheading of several westerners.
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