The Islamic State is a growing threat to northeast Syria, and the group will again flourish unless immediate action is taken, the Kurdish-led region’s security chief said in the wake of last month’s deadly prison attack. Mazloum Abdi, the commander of the U.S.-backed Syrian Democratic Forces, said immediate security measures were taken to contain active IS sleeper cells, but the group is proving to be a resilient insurgency. The threat remains high, he said, despite the death of the group’s leader in a U.S. commando operation last week. “We are surrounded by the Islamic State,” Abdi said durin…