The United States’ focus on the Islamic State has given cover to Syrian forces, they say. That has freed Mr. Assad’s military from worrying about checking the militant group’s advances and allowed them to continue to focus attacks on the greater political threat — less extreme Syrian-based insurgent groups bent on ousting Mr. Assad and the communities where they hold sway.

The Syrian government had long focused its attacks on insurgents other than the Islamic State, a group that had seemed more interested in establishing Islamic rule in its territories than in ousting Mr. Assad. But after the group overran parts of Iraq and carried out a series of lightning routs of Syrian Army bases, terrifying many government supporters, Syrian warplanes began attacking it with more intensity in its eastern strongholds.

Since the American-led campaign began about two weeks ago, however, the need for Syrian forces to check the Islamic State has ebbed, and some insurgents who oppose the militant group say the government attacks on them have intensified.