By Opheera McDoom

KHARTOUM, July 19 (Reuters) - More than 50 people were killed and 20 injured in Sudan's worst floods in living memory which have partially or completely destroyed 18,000 homes, the head of civil defence said on Thursday.

Hamadallah Adam Ali told Reuters major roads to some parts of the country had been flooded and police helicopters and government planes were flying in emergency aid and tents to affected areas in Sudan's east, southeast and around Khartoum.

"It seems more than 50 have been killed, but less than 75," he said. "There are more than 20 injured in the hospitals," he said, adding these numbers could rise.

Ali said these were the worst floods he had seen in Sudan. "One man who was about 90 years old told me he had never seen waters like these in his entire life," Ali added.

He said 18,000 houses had been affected.

Rains, flash floods and overflowing rivers forced hundreds of families to seek main roads, often the highest ground around, as they watched houses and possessions get washed away.

Ali blamed climate change and countries who have been polluting the environment for worsening rains.

Aid workers have said with better monitoring and planning Sudan's authorities could prevent deaths.

The civil defence authority said people build in high-risk areas close to river banks or in flood-prone plains, ignoring government warnings.

A rainy season from June to September each year in Sudan causes floods, especially in the east. Sudan is mostly desert at other times of the year and uses little of the river Nile waters allocated to it under an east African treaty for agriculture.

Levels of the Nile in Khartoum last year were higher than in 1988 and 1946 when the worst floods of last century hit Sudan. Last year some 10,000 houses were destroyed during about four months of rains.

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