The research, from Minority Rights Group International (MRG), placed Somalia first, Iraq second and Sudan third in its "Peoples Under Threat" list.
Although Afghanistan came fourth and Myanmar fifth, African states made up more than half of the countries in the top 20.
"A new government in Somalia has raised hopes for democracy, but it is also a uniquely dangerous time," said Mark Latimer, director of MRG.
"There is the spectre of a return of large-scale clan violence -- and groups that supported the old order are now under tremendous threat."
The organisation mentions the Darood, Hawiye and Issaq clans as being under particular threat, as well as the Bantu group.
Somali forces loyal to the fledgling administration have been battling a violent insurgency since they drove a powerful Islamist movement out of southern and central areas in January with Ethiopian support.
African Union peacekeepers are arriving to facilitate the departure of Ethiopian troops and a takeover by So