GENEVA - U.N. aid agencies will launch an emergency appeal to raise for about $550 million to help survivors of the earthquake in Haiti, a U.N. spokeswoman said on Friday.

Thousands of people injured in Tuesday's massive earthquake in the Caribbean country spent a third night waiting for help, many lying on sidewalks, as their despair turned to anger.

The reality is that getting the quantities of supplies, equipment and expertise that are so desperately needed on the ground inevitably takes time, U.N. Emergency Relief Coordinator John Holmes said in a statement.

The world body also said it was looking at converting the national soccer stadium in Port-au-Prince into a field hospital and at setting up collective kitchens for the homeless.

More than 25 search and rescue teams are now deployed at schools, hotels, hospitals and larger buildings in the capital, with 13 more on their way, according to the U.N. statement.

There are pockets of survival, we shouldn't give up hope, said Elisabeth Byrs, spokeswoman of the U.N. Office for the Coordination of Humanitarian Affairs (OCHA). They are working around the clock.

No further field hospitals were required but medical teams including surgeons and medicines were badly needed, she said.

At least 10 percent of housing in the capital was destroyed, making about 300,000 homeless, but in some areas 50 percent of buildings were destroyed or badly damaged, according to a preliminary assessment by U.N. disaster experts.

Under the U.N. appeal, the World Food Programme will seek to provide life-saving food rations to 2 million destitute people for the next month. A longer-term operation is planned up to July 15.

We need high-energy biscuits and ready-to-eat meals as quickly as possible, WFP spokeswoman Emilia Casella said.

After initialing citing reports from partner agencies that its warehouses had been looted, the WFP later said it reached the Port-au-Prince warehouse in question and stocks were safe.

Our warehouse has not been looted. We have 6,000 tons of food there and it is being protected by the Brazilian military, Caroline Hurford, a WFP spokeswoman in London, told Reuters.

The WFP distributed food to 4,000 people gathered at the prime minister's compound in Port-au-Prince on Thursday following an earlier hand-out in the town of Jacmel.

We are trying to get the food we do have our hands on to people. What we have been able to do so far is a drop in the bucket, Casella said.

The WFP was also exploring setting up 200 collective kitchens in Port-au-Prince to feed the homeless, she said.

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