KAMPALA, Oct 17 (Reuters) - The United Nations' World Food Programme has been forced to air drop food for the first time in Uganda after the east African nation suffered its worst flooding in 35 years.

In a statement released late on Tuesday, the WFP said the operation was a desperate last resort to help tens of thousands of people after flooding washed away vital roads.

"Resorting to food airdrops reflects the severity of the heavy rains and floods ... the worst in 35 years," WFP Uganda acting country director Alix Loriston said.

"There is simply no other way to get survival rations to isolated people."

Uganda has been one of the countries worst hit by torrential rains and flash floods that swept over east and west Africa in the past few months, washing away villages, destroying food crops and killing animals.

Conservative estimates put the total number of people killed, from Ethiopia to as far west as Senegal, at 200. Aid agencies say at least a million people have been affected and expect the death toll to rise.

"Flooding has affected 300,000 people in northern and eastern Uganda ... tens of thousands ... in the north are unreachable," the WFP said.

When compared with some of its drought-prone neighbours, Uganda is often seen as a land of plenty -- full of green fields fed by regular rains.

But meteorologists say its weather has become unpredictable with unprecedented spells of drought followed by floods. Some blame climate change caused by greenhouse gas emissions.

The WFP said most food drops were in the north, ravaged by two decades of civil war that have made it long dependent on food aid usually delivered by roads now impassable.

"Some 250,000 people ... in northern Uganda did not receive September rations," the statement said.

The agency said it urgently needs around $20 million for food and trucks to transport it.

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