Gunmen from the Ras Kamboni militia of former Islamist warlord Ahmed Madobe -- recently self-appointed "president" of the southern Jubaland region -- battled against forces loyal to Iftin Hassan Basto, another leader claiming to be president.
Fighting broke out yesterday evening, paused overnight, but resumed on today morning.
"Fighting started when soldiers from Ras Kamboni attacked and tried to arrest me," Basto told reporters. "But my men fought back and defended me."
Several rival factions claim ownership of Kismayo, a former stronghold of the Al-Qaeda-linked Shebab, where Kenyan troops in an African Union force are now based.
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Witnesses said eight were killed during the clashes, with several wounded people were seen being taken to hospital.
"I saw eight dead, three of them civilians, but the toll could be higher as many were wounded," said Mohamed Farey, a witness.
"Battles have continued... We can hear heavy fire."
Another resident, Jama Bile, said that three of his neighbours had been killed, and two others wounded.
Jubaland lies in the far south of Somalia and borders both Kenya and Ethiopia, and control is split between multiple forces including clan militia, the Shebab, Kenyan and Ethiopian soldiers.
Jubaland joins other semi-autonomous regions of the fractured Horn of Africa nation, including Puntland in the northeast -- which wants autonomy within a federation of states -- and Somaliland in the northwest, which fiercely defends its self-declared independence.