Clashes between Syrian rebels and their rivals from an al-Qaeda-linked faction spread on Monday from the country’s opposition-held areas in the north to a key eastern city, activists said.

The rebel-on-rebel fighting in the eastern city of Raqqa — a long-time bastion of an al-Qaeda-linked group known as the Islamic State in Iraq and the Levant — reflects a widening war within a war in Syria, this one against radical extremists.

It also suggests emboldened rebels are trying to completely overrun their al-Qaeda rivals. The infighting has been the most serious since armed groups initially rose to try overthrow the rule of Syrian President Bashar Assad.