About a third of rebel groups fighting in Syria share the aims and ideology of Islamic State, says a new report into the conflict in the region.
It looked at the views of 48 Syrian rebel factions, and found 33 per cent follow Isis's ideology, Sky News reports.
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The Islamist militias that hold similar religious beliefs and political goals to the so-called Islamic State group are believed to contain 100,000 fighters and could grow to fill the void should IS be defeated, says the study. The report states: "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.
Current Western efforts to define "moderate" and "extremist" rebels are also bound to fail, says the centre, because the groups themselves rarely make the distinction.
The report comes in the wake of Friday's UN Security Council resolution which unanimously endorsed a peace plan for Syria, including calls for a ceasefire. "The resolution sets out a timetable for formal talks and a unity government within six months," reports the BBC, but it "makes no mention of the future role of President Assad".
The centre's report claims that unless President Assad is removed from office or voluntarily steps down, the war in the country is likely to remain or could even spread further.
Western countries have called for his departure, but Russia and China say he should not be required to relinquish power as a precondition for peace talks.