More than 10 people were killed in clashes on Monday in Libya between Toubou tribesmen and residents of the southern city of Sabha, local officials said.
"More than 10 people were killed today in violent clashes between the Toubou and residents" of Sabha, Abdelrahman Seif al-Nasr, security chief of the southern Fezzan region, told AFP.
According to Ali al-Dib, a former rebel, the clashes erupted in the city centre and resulted in between 15 and 20 dead among the ex-rebel forces.
"We have taken back the offices of the military council which were seized by the Toubou. The fighting is continuing and getting worse," he said by phone from Sabha, with the noise of automatic gunfire clearly audible in the background.
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Dib said the fighting broke out when the Toubou refused to hand over to local authorities one of their men accused of killing a man belonging to the Bussif tribe.
Mohammed al-Harizi, spokesman for the ruling National Transitional Council, was unable to give any further details when contacted by AFP.
However NTC member Mukhtar al-Jadal confirmed there had been fighting in Sabha, an oasis city in southern Libya, and said that NTC chief Mustafa Abdel Jalil had met representatives from the south in an effort to broker a solution.
A local source said Toubou tribesmen who had been in "brigades of former rebels have defected and rejoined their own" people, adding that "some elements from Chad are fighting with the Toubou."
Toubou tribesmen have also been involved in deadly clashes with another tribe in the Saharan oasis of Kufra, where two ethnic groups are locked in a standoff over smuggling.
The Toubou are traditionally oasis farmers who also have connections beyond Libya's borders.