The long-term aim would be to defeat or demoralise the non-Isil opposition, so that Isil became the regime’s only enemy. That would force the West to back President Bashar al-Assad against it.
“They want to clean the country of non-Isil rebels, and then the US will work with them as Isil will be the only enemy," the Damascus source said.
ISIL uses the so called moderate rebels to hide behind.
These are the moderate rebels
http://www.liveleak.com/view?i=4d0_1354450929
the only difference between them and ISIS is they have better PR