The United Nations has urged the authorities in southern Somalia to arrest the gunman who killed a UN doctor earlier this week - the latest in a succession of attacks on aid workers.

Dr Ayub Sheikh Yerow was a much respected figure in southern Somalia.

As the project officer for health and nutrition in the UN children's organisation, Unicef, he was a key member of staff helping to deliver emergency assistance to thousands of children and women affected by drought.

He was shot by a gunman who ambushed his vehicle between the towns of Jowhar and Afgoi, and later died in hospital.

A Unicef statement said Dr Ayub's death was one more on a growing list of casualties involving humanitarian workers trying in extremely dangerous situations to help the Somali people.

Last month a local aid worker with the Spanish branch of the medical relief agency Medecins sans Frontieres was killed in a similar way, by a gunman in an ambush.

Impunity

Although the UN has appealed to the authorities to arrest Dr Ayub's killers, the problem is that in Somalia it is often very difficult to know who is in charge of a certain region.

A recent UN report on the situation in Somalia said that much of the violence nowadays is carried out by bandits who have easy access to guns and who can rob with impunity.

The sort of vehicle in which Dr Ayub was travelling is much coveted by the militiamen.