More than 100,000 of the 160,000 Somalis at Dadaab have been displaced -- up by 20,000 from the previous figure -- and many are moving to drier areas, accompanied by donkey carts with their meager belongings, it said.

While the water has receded somewhat since the last flash flood on Saturday, the UN refugee agency (UNHCR) said more buildings and shelters had collapsed due to the rains and warned of a major threat of waterborne diseases.

After five planeloads of emergency relief supplies were delivered to the worst hit camps of Dagahaley and Ifo on Sunday and Monday, the UN refugee agency began relocating the most-affected from Ifo to the third site, Hagadera, it said.

"Because of the difficulty of road transport in the flooded region, able-bodied men are being encouraged to walk to Hagadera, while most women and children are carried by truck," the UN High Commissioner for Refugees said.

About 2,000 refugees, along with 32 donkey carts carrying their scant possessions, had made the 20-kilometer (12.5-mile) trek by Monday night, the agency said, adding that at least 250 more were waiting to move.

It said the airlifted aid -- made necessary after floods that have killed at least 31 people in Kenya in the past three weeks cut the only supply route to Dadaab from Nairobi -- was now being distributed in appalling conditions.

"Health and sanitation remain a serious concern," UNHCR said in a statement. "The general health of the population has deteriorated due to the conditions, lack of food and sleep and difficulty in accessing medical care."

Spillage from latrines in the camps has contaminated standing water and poses grave risks, it said, adding that fever, diarrhoea, and eye and skin infections have become common complaints among refugees.

The flooding has compounded the misery of the refugees, many of whom have fled from insecurity, drought and fears of an all-out war in Somalia between the country's weak government and powerful Islamist movement.

The UN special envoy for Somalia Francois Fall urged a speedy delivery of humanitarian supplies for at least 50,000 people who have been displaced by floods in the lawless African nation.

Fall lamented that the response was slow despite a weekend appeal by Prime Minister Ali Mohamed Gedi for humanitarian aid to avert a major disaster there.

While at the seat of government in Baidoa on Monday, Somali leaders "expressed concern that too little relief was reaching people in dire need", Fall said in a statement released in Kenya.

Fall said UN agencies and other humanitarian groups were delivering emergency supplies to the affected people, "but more will be needed" in order to avert suffering caused by unusually heavy rains that have caused the worst floods in 50 years.

Flooding in southern and central Somalia has claimed at least 52 lives and affected more than 1.5 million people, most of whom were weakened by a recent drought that affected around 15 million across east Africa.

Meanwhile, the London-based Muslim Aid launched an appeal seeking to raise one million dollars to help hundreds of thousands of displaced people in the worst-hit Horn of Africa region, it said in a statement.

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