"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 Organization (WHO) said on Friday.

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

Clashes have put on hold a vital mass polio vaccination campaign, after the first cases were confirmed six years after Somalia was declared free of the crippling virus.

"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 are battling for control of Kismayo, a strategic and economic hub in the southern Jubaland region. They include former extremist chief Ahmed Madobe, who in May appointed himself "president" of Jubaland, and Bare Hirale, a former Somali defense minister who also leads a powerful militia.

Mogadishu has accused Kenyan troops of encouraging the clashes in Kismayo.

Somalia's government has demanded that Kenyan troops stationed in Kismayo as part of an African Union force be replaced, accusing them of backing Madobe's Ras Kamboni militia, which opposes Mogadishu's rule.

Kenyan troops, which invaded Somalia in 2011, fought alongside Madobe's troops to oust al-Shebab fighters from Kismayo, before joining the 17,700-strong AU force.

The AU force is mandated -- and funded by the UN and European Union -- to support central government.