"The region is going through decisive times" and the most important factor in determining the outcome is whether there is "real and sincere international cooperation" against the jihadists, Assad said.
"Terrorist groups, led by IS, did not emerge from nothing but are the fruit of the mistaken and aggressive police of those who have waged war on Syria," state news agency SANA quoted him as saying.
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Assad was speaking to members of his ruling Baath party, just days after US President Barack Obama rejected any alliance with the Syrian president against the jihadists.
Obama, who leads an international coalition against IS, which controls swathes of Syria and Iraq, said in Australia on Sunday that Assad, who is facing a nearly four-year armed uprising, "has lost legitimacy with the majority of his people".
"For us to then make common cause with him against ISIL (Islamic State) would only turn more Sunnis in Syria in the direction of supporting ISIL and would weaken our coalition," he said.