"Violent confrontations broke out between Toubous and Awled Sleiman early this morning," Ayoub al-Zarrouk, chief of the local council in Sebha told AFP. "So far there are 19 dead and 20 wounded."
Local sources said the clashes were sparked by the death on Thursday of a militia chief linked to Awled Sleiman, adding that the tribe accused the Toubous of murdering him.
Today's fighting is the worst between the tribes since they struck a cease-fire agreement in March 2012 following deadly battles that killed at least 150 people and wounded 400 others.
The Toubous are black oasis farmers by tradition who also live in southern Libya, northern Chad and Niger.
They have complained in the past of attempted ethnic cleansing against their community by Arab tribesmen.