In the countryside northeast of Aleppo on Tuesday, Islamic State fighters fought rival Syrian insurgents amid fears that the Islamic State was positioning itself to make Aleppo its next big prize. Syrian opposition leaders accused the Syrian government of essentially collaborating with the Islamic State, leaving the militants unmolested as they pressed a surprise offensive against other insurgent groups — even though the government and the Islamic State are nominal enemies — and instead striking the rival insurgents…

Neither American officials nor Syrian insurgents have provided proof of such direct coordination, though it has long been alleged by the insurgents. The State Department spokeswoman Marie Harf told reporters Tuesday that United States officials were looking into the claims but had no independent confirmation.

What is clear is that Mr. Assad and the Islamic State reap benefits by eliminating or weakening other insurgent groups. Mr. Assad can claim he is the only alternative to the Islamic State, and the Islamic State can claim it carries the banner of oppressed Syrians and Iraqis.

But insurgents said the recent fighting around Aleppo provided by far the strongest indication of active coordination. “It was never this blatant,” said Abu Abdo Salabman, a spokesman for the Sham Revolutionary Brigades, a rebel group that has sent reinforcements to the battle.