Thousands of fearful Rohingya Muslim and Rakhine Buddhist civilians fled the worst fighting to grip Myanmar's northwest in five years, with 104 people killed and the United Nations and international aid groups forced to pull out some staff.

The mass escape from the northern part of Rakhine state was triggered by widespread coordinated offensives by Rohingya insurgents wielding sticks, knives and homemade bombs in attacks Friday on 30 police posts and an army base.

The violence marks a dramatic escalation of a conflict that has simmered in the region since last October, when a similar but much smaller Rohingya attack prompted a brutal military response dogged by allegations of human rights abuses.