DAKAR, Aug 10 (Reuters) - Seasonal flooding has killed almost 100 people across West Africa, destroyed thousands of homes, encouraged the spread of disease and threatens to worsen a food crisis by washing away farmland, aid groups said on Tuesday.

Millions of people are without food in the Sahel region, which runs south of the Sahara desert, after droughts last year depleted stocks.

The region depends upon October harvests which need rain, but heavier-than-normal downpours are counterproductive because they can break the planting cycle and ruin farmland in Chad and Niger, the countries worst affected by shortages.

"Rain in the Sahel is much welcome but it needs to be properly distributed over time and over space which is the major issue now," said Naouar Labidi, the U.N. World Food Programme's regional food security adviser.

The river Niger burst its banks at the weekend, destroying hectares of vegetable gardens and rice fields and displacing at least 5,000 people around the capital Niamey, the U.N office for the coordination of humanitarian affairs said in a report.

Another 20,000 are at risk of displacement in the event of further heavy rains, which could make life even tougher.

"The nutritional situation is becoming much worse than last year," said a report by aid agency USAID and Famine Early Warning Systems Network released on Tuesday. In some regions, cases of acute malnutrition had doubled since 2009, it said.

Heavy Rains

Elsewhere in West Africa, heavy rains have caused death and damage in other ways.

In Sierra Leonean capital Freetown, 16 people died at the weekend when a slope on which their homes were built crumbled in a landslide after heavy rains, local police said.

"This disaster occurred because people built houses in high risk areas which have been declared uninhabitable by the government," Ahmed Ba, an official at Sierra Leone's office for disaster management, told Reuters.

In northern Cameroon, water-borne disease cholera has killed more than 150 people in the past two months.

"Since the first case was diagnosed in the region in early June, hardly a day passes without new cases recorded," said the regional delegate for public health Rebecca Djao.

"The situation has been aggravated by heavy rainfall, flooding, collapse of pit toilets and contamination of streams which are the main source of drinking water."

Seasonal floods and mudslides regularly cause destruction and death across west Africa. Last year they drove over half a million people from their homes and killed about 190 others.

Experts say abnormal weather linked to climate change could be the cause of heavier than normal precipitation, but they also blame poor town planning and overcrowding of cities for the damage done when rains do fall.