* Street battles erupt for second day, five killed on Friday
* Kismayu a litmus test after decades of Somalia conflict
* Renewed insecurity may benefit weakened Islamist militants
Gunbattles between militia groups in the southern Somalia's Kismayu erupted for a second day on Saturday, residents said, in the heaviest fighting the disputed port city has seen in more than four years.
The fighting, the first since several former warlords staked rival claims on the lucrative port and fertile hinterlands in May, has stoked fears among locals of a return to the clan wars that tipped the country into anarchy two decades ago.
One resident counted five dead militiamen on Friday. It was not clear if there were casualties from Saturday's fighting.
"From my rooftop I can see dozens of men with Ak-47s and their technicals exchanging fire in the streets," shopkeeper Farah Nur said, referring to the machine gun-mounted pick-up trucks commonly used in Somalia.
Businesses stayed shut and the streets of Somalia's second biggest city were empty of civilians as mortar blasts rang out.
Kismayu was controlled by Islamist al Shabaab until last September when they fled an offensive by Kenyan peacekeeping troops supported by a militia group loyal to a former governor of Kismayu, Ahmed Madobe.
A local assembly last month declared Madobe president of the southern Jubaland region, handing him back control of Kismayu.
But Somalia's central government in the capital of Mogadishu, which does not view Madobe favourably, said the process was unconstitutional. Within days, three other men had pronounced themselves president, including Barre Hirale, a former warlord and defence minister seen as pro-government.
The latest clashes broke out when Madobe's fighters stopped another of the claimants from visiting a hotel were Somalia's defence minister and other officials were meeting.
Regional capitals and Western donors are nervous of any reversal of security gains made in Somalia by African Union peacekeepers in the fight against the al Qaeda-linked militants, seen as a threat to stability in east Africa and beyond.
How the fate of Kismayu and the Jubaland region is resolved is a litmus test for Somalia as it rebuilds from the ruins of war and cements a fragile peace.
Mogadishu insists there is no going back to civil war. But government-led talks over Kismayu are being stymied by the divisive clan politics that plague Somalia.
"We hope fighting will cease. Only the man with the most weapons will remain in power," local elder Nur Ibrahim told Reuters by telephone, showing scant hope for a negotiated end.
Madobe's apparently close relationship with the Kenyan military has raised tensions between the Mogadishu and Nairobi governments. A Kenyan ally in southern Somalia could provide Nairobi with a welcome buffer along their porous border.
On Saturday, Somalia's President Hassan Sheikh Mohamud returned to Mogadishu after talks with Kenya's new leader. The pair have met at least five times since Uhuru Kenyatta's inauguration in early April, aides say.
Al Shabaab blamed Kenya for the latest violence.
"The resurgence of tribal hostility in Kismayu is a result of the Kenyan invasion of the city and the Kenyan government will be held fully responsible for every drop of blood," Sheikh Xudayfa Abdirahman, a senior al Shabaab official, told Reuters.
Al Shabaab wants to impose a strict version of Islamic law on Somalia and has been blamed for many attacks on Kenyan soil.