About 60% of fighters in rebel factions in Syria identified with a religious and political ideology similar to that of the terror group, it added.
The thinktank, run by the Tony Blair Faith Foundation, said: “The west risks making a strategic failure by focusing only on IS. Defeating it militarily will not end global jihadism. We cannot bomb an ideology, but our war is ideological.”
The report comes after the United Nations agreed a resolution endorsing the start of “urgent” formal negotiations between Assad’s regime and moderate opposition groups early next month.
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But the centre warned the radical groups, including al-Qaida affiliate al-Nusra Front and Ahrar al-Sham, could benefit if they went “unchallenged”.
It added: “If Isis is defeated, there are at least 65,000 fighters belonging to other Salafi-jihadi groups ready to take its place.
“The greatest danger to the international community are the groups that share the ideology of Isis, but are being ignored in the battle to defeat the group.
“While military efforts against Isis are necessary, policy makers must recognise that its defeat will not end the threat of Salafi-jihadism unless it is accompanied by an intellectual and theological defeat of the pernicious ideology that drives it.”
The Centre on Religion and Geopolitic is a think tank that is part of the “Tony Blair Faith Foundation"