By Abdi Sheikh and Aweys Yusuf

MOGADISHU, March 28 (Reuters) - Somalis uprooted by fighting in Mogadishu looted trucks carrying U.N. food aid on Friday, peacekeepers said, highlighting what relief agencies warn is a fast deteriorating humanitarian catastrophe.

Somalia now has one million internal refugees, aid workers say, and their numbers are swelled by an exodus of some 20,000 civilians each month from the capital, where Islamist insurgents are battling the Ethiopian-backed government.

Captain Clement Cimana, spokesman for a small African Union peacekeeping force in the coastal city, said the displaced residents targeted trucks carrying supplies for the U.N. World Food Programme (WFP) before local police restored order.

"They also blocked the main road, showing their anger," he told Reuters. "They said they always see WFP-chartered trucks full of food passing in front of them while they are hungry."

Aid agencies say record high food prices, hyper-inflation and drought across the country are exacerbating the crisis and will worsen if seasonal rains due next month fail as expected.

Meanwhile, police and witnesses in Merka, south of Mogadishu, said a small unmanned plane had crashed near the coast. Local media speculated that it was a U.S. surveillance drone controlled from a warship in the Indian Ocean.

"I saw it myself. It's equipped with a camera and what looks like a small computer," said local man Yussuf Ahmed. "It was made of plastic and about 1.5 metres long."

Merka police chief Osman Hassan Hussein said his forces had secured the crash site: "We were afraid it could explode. There was gas leaking from it and a crowd of people had gathered."

The U.S. military has launched several air strikes in Somalia in recent months, targeting al Qaeda suspects including the bombers of the U.S. Embassies in Kenya and Tanzania in 1998.

Remnants of a hardline Islamist administration that was driven from Mogadishu in late 2006 are blamed for an Iraq-style insurgency of assassinations and roadside bombings that killed 6,500 people last year in Somalia's capital alone.

The violence, including attacks on humanitarian workers, has limited access to victims, 40 aid agencies said this week.

The situation continues to deteriorate, the agencies said in a statement, and the 250,000 civilians camped between Mogadishu and Afgoye to the west are now considered the biggest group of internally displaced people in the world. (Additional reporting by Ibrahim Mohamed in Baidoa; Writing by Daniel Wallis) (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/)

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