Mogadishu: Gunmen shot dead three Somali elders on Friday who were helping local aid workers distribute food in a refugee camp outside Mogadishu, witnesses said.

The shooting, by unknown men, appeared to be the latest deliberate attack on humanitarian efforts in the Horn of Africa nation where an insurgency is raging and more than one million people live as internal refugees.

Witness Abdul Kafi Hassan said one of the elders died on the spot, while the other two passed away in hospital.

"We suspect the elders were killed by the same gunmen who have been targeting aid workers," he added of the three, who were local leaders among the refugee community.

A wave of assassinations of senior local humanitarian workers has shocked aid agencies and forced many to consider suspending operations. Four foreign aid workers, two Italians, a Kenyan and a Briton, are also being held hostage.

Suspicion has often fallen on insurgents, whom the government says have links to Al Qaida.

The African Union (AU) has 2,200 peacekeepers in Somalia, but they have done little to stem the violence and the continental body wants to hand over to the United Nations.

In typical violence rocking Mogadishu virtually daily, a stray mortar landed on a shack on Thursday, killing a mother and her three sons, during an insurgent attack on Ethiopian troops, who are supporting the Somali government, witnesses said.

"The mortar shell melted their house made of iron sheets. The father and three other children sustained light injuries," witness Ahmad Mohammad said. At least four insurgents also died.

Shaikh Abdul Rahim Eisa Adow, a spokesman for the insurgents, said only two of their fighters had died whereas "many" Ethiopians had been killed.

The UN warned on Friday that attacks on aid workers and threats from pirates to ships delivering food were jeopardising the lives of millions in need.

"Somalia is at a dire crossroads," Peter Goossens, Somalia director for the UN World Food Programme, said in a statement.

"If sufficient food and other humanitarian assistance cannot be scaled up in the coming months, parts of the country could well be in the grips of disaster similar to the 1992-1993 famine, when hundreds of thousands of people perished."

WFP said insecurity was exacerbating an already dreadful situation of failed harvests and high prices.