UN head Ban Ki-moon is visiting the Somali capital, Mogadishu - the highest-ranking foreign official to go there in years.He was wearing a bulletproof vest as he was welcomed at the airport by Somalia's prime minister.His visit comes a day after the fiercest clashes in the city for several months.Islamist militants are battling the forces of the UN-backed government and African Union troops.Somalia has been racked by war for two decades and has not had a functioning national government since 1991.