GENEVA, July 18 (Reuters) - The United Nations appealed on Wednesday for $38 million to help hundreds of thousands of people whose homes, crops and livestock were destroyed by severe floods in southern Pakistan.

Many of the 2.5 million people affected by the flooding, which followed four days of torrential rains from a cyclone, are living in makeshift settlements, camps and public buildings, U.N. Emergency Relief Coordinator John Holmes said.

The funds are to be used to provide urgent assistance including shelter, water and medical care, and help communities reeling from damaged food stocks and farmlands to recover, Holmes said.

"If we don't act quickly their plight is likely to deteriorate further," he said in the appeal.

Separately, the International Federation of the Red Cross revised up its appeal for Pakistan to $17 million, from an initial request of $8.5 million, and said ongoing rains and moving flood waters were threatening new villages in the region.

"Shelter is still a major issue," Asar Ul Haq, the International Federation's head of disaster management in Pakistan, said in a statement.

"Some people have made improvised shelters using raised bed frames, while others are sleeping outdoors, and of course people are worried that the heavy rains will continue," Ul Haq said. "We're looking very carefully at how best to meet their near- and longer-term needs."

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