Kerry, hosting a meeting of an anti-IS coalition in Brussels, said international airstrikes were finally stopping the advance of the jihadists across Iraq and Syria, but warned it could take years to finally defeat them.
But in a sign of the deepening complexity of the regional conflagration, Syria's Iranian-backed President Bashar al-Assad criticised the Western and Arab air strikes for having no effect.
Kerry told the meeting of officials from 60 states in the coalition that a campaign of around 1,000 strikes had had a "significant" impact on the Sunni extremist IS, which declared a caliphate in Syria and Iraq in June.
He denied there was any military coordination with Iran, after the Pentagon said earlier that Iranian F-4 Phantom jets -- acquired from the United States before the 1979 Islamic revolution -- had deployed against IS fighters in eastern Iraq's Diyala province.
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But he suggested there is a tacit understanding between mainly Shia Iran and the United States to tackle a common threat.
"If Iran is taking on (IS) in some particular place... and it has an impact, then it's going to be net effect (that) is positive," Kerry told a press conference after the meeting.
The coalition issued a statement saying that the militant group's "advance across Syria and into Iraq is being halted", and that Iraqi and Kurdish forces were reclaiming territory.
They also agreed to develop a "multifaceted" strategy to combat IS, including stopping the flow of foreign fighters, cutting finance and "delegitimisation" of its powerful, social media-driven brand.
In Paris, French President Francois Hollande said today his country was also ready "to step up actions" against IS militants in Iraq. He made the comments in a joint statement with visiting Iraqi Prime Minister Haider al-Abadi.