Mogadishu - Unknown Somali gunmen have killed a senior worker with the United Nations Children's Fund (UNICEF) in the south-west of the conflict-ridden Horn of Africa nation, witnesses said Monday.

It was the second killing of a UN employee in three days.

Muktar Mohamed Hassan, the head of UNICEF's water and sanitation programme, was killed by men armed with pistols in Hudur, about 430 kilometres south-west of the capital Mogadishu, on Sunday night.

"They fired three bullets into his head," witness Mohamed Mohidin Hassan told Deutsche Presse-Agentur dpa. "He died on the spot."

The assailants fled after the attack.

A senior officer of UN World Food Program (WFP) was killed Friday as he left a mosque in the central town of Merka.

Aid workers have increasingly become targets for murder and kidnap this year as Somalia's brutal insurgency shows no signs of letting up.

Almost 10,000 civilians are believed to have died in the insurgency, which broke out in early 2007 after the government and its Ethiopian allies kicked out the Islamist regime.

The Horn of Africa nation has been plagued by chaos and civil war since the ouster of dictator Mohamed Siad Barre in 1991. (dpa)