GAZIANTEP, Turkey — The fighters with the Free Syrian Army were expecting an attack any day from the jihadists besieging the city of Minbej in war-torn Syria, fortifying their base, once a carpet factory, with concrete bomb-blast barriers.

But they did not suspect the teenagers pushing a broken-down sedan past the front gate. Then a boy who looked no older than 14 blew up the car and himself, unleashing an assault that killed or wounded nearly 30 rebel fighters and ultimately put all of Minbej under the control of the most extremist jihadi group in the Syrian conflict.

“They call us godless,” said Sheikh Hassan, the leader of the Free Syrian Army brigade that came under attack. “They attack us from the front, they attack us from the back.”

That battle was one snapshot of the rise of the Islamic State of Iraq and Syria, or ISIS, a militant Sunni group whose thousands of fighters have occupied crucial swatches of Syria and have now surged into northern Iraq. The group has vowed to create a caliphate spanning the Sunni-dominated sections of neighboring countries.