BEIRUT, Lebanon — An internal war between Syrian insurgents and their onetime allies, members of a transnational jihadist group that even some fighters affiliated with Al Qaeda reject as too extreme, widened on Monday from parts of northern Syria into Raqqa, the largest city in eastern Syria that had been under the group’s control, antigovernment activists and fighters said.

Some activists said the group, the Islamic State of Iraq and Syria, had been largely evicted from Raqqa, which it had ruled for months. The group was known to have closed churches, arrested hundreds of Syrians who disagreed with its goals and even executed some perceived offenders of strictly interpreted Shariah law, the code of Islamic behavior it had sought to impose.

If confirmed, its expulsion from Raqqa would be a setback for the group, known by the initials ISIS, in its effort to assert supremacy over the nearly three-year-old insurgency in Syria, which has devolved into a splintering of militias with no universally recognized authority.

“The initiation of heavy fighting in Raqqa was perhaps the most important signifier of importance,” said Charles Lister, a visiting fellow at the Brookings Doha Center in Qatar. He said that given the size and location of Raqqa, coupled with its importance as a jihadist stronghold, “should ISIS be forced out there, it will have been dealt an extremely symbolic blow.”