Here are some key facts about the Ogaden National Liberation Front (ONLF):
* Formed in 1984 amid a resurgence of separatist sentiment in the Ogaden region on Ethiopia’s border with Somalia, many of its first members supported Mogadishu in its failed war with Addis Ababa over the region in the late 1970s.
* Ethiopia accuses the ONLF of being terrorists supported by arch-foe and neighbour Eritrea.
* The ONLF’s aims have varied over time, ranging from full-scale independence to joining a "Greater Somalia", to more autonomy within ethnically diverse Ethiopia.
* ONLF fighters, who do not wear uniforms, have taken advantage of their close ties to the area’s largely nomadic communities, crossing expanses of open land to launch hit-and-run attacks on Ethiopian military convoys. They often melt into villages and hide among herders when counter-attacks are threatened.
* The Ogaden region is almost entirely populated by Muslim, Somali-speaking people. The region has kept its own distinctive identity, doing the bulk of its trade with Somaliland, Somalia and the Middle East rather than the rest of "highland" Ethiopia.
* The separatist cause has been fuelled by widespread resentment at the region’s low level of development. Until Chinese engineers starting moving in late last year, the entire region could only boast just over 30 km (20 miles) of tarmac road, all of it around the regional capital Jijiga. The area has also been battered by a succession of severe droughts and floods.