Battles between rival warlords in Somalia's key southern port city of Kismayo have killed at least 71 people last month, UN officials have said.

"Recent fierce fighting... continues to have a profound impact on civilians and humanitarian aid work in the Lower Juba region," the UN's World Health Organisation (WHO) said on Friday, adding the clashes also left more than 300 injured last month.

The WHO, which supports hospitals treating the war-wounded in Kismayo, said injuries and deaths outside the hospital are estimated to be much higher but cannot be confirmed.

"Kismayo remains a volatile area, with observed increase in fighting among warring factions, and other incidences of violence such as landmines and hand grenade attacks," the WHO added.

Several rival factions claim ownership of Kismayo, where Kenyan and African Union forces are now based after driving out the al-Qaeda-linked al-Shabab group.

They include Ahmed Madobe, the chief of armed group Ras Kamboni, who in May appointed himself "president" of Jubaland, and Bare Hirale, a former Somali defence minister who also leads a powerful militia.

Kismayo has changed hands more than a dozen times since the collapse of the central government in 1991.

The city was controlled by al-Shabab until last September when the armed groups fled an offensive by Kenyan troops supported by Madobe's Ras Kamboni.

A local assembly last month declared Madobe president of the southern Jubaland region, handing him back control of Kismayo.

But Somalia's central government, which does not view Madobe favourably, said his appointment was unconstitutional.

Within days three other men had pronounced themselves president, including Barre Hirale, a pro-Mogadishu former defence minister.

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 fighters, seen as a threat to stability in east Africa and beyond.