At least four people have died and more than 20,000 have been affected by floods in Niger caused by weeks of heavy rain, the United Nations said Friday.

The bad weather has destroyed 2,170 homes and flooded nearly 545 hectares of agricultural land, the UN's Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement.

Around 3,100 displaced victims are currently living in schools or have been taken in by host families, the agency said citing Niger's emergency civil protection services.

The bad weather is showing no sign of abating as the rainy season continues in the impoverished sub-Saharan country, which has often faced severe food crises due to flooding or drought.

Since late July, local media have reported repeated floods, particularly in central and southern Niger.

In the capital Niamey, the authorities have asked residents living near the edge of the Niger River to leave their homes because of the flood threat. The water level is at 580 centimetres (19 feet), well above the "alert" threshold of 530 centimetres, according to the Niger Basin Authority (NBA).

Of the eight regions of Niger, only Diffa in the east, which is home to more than 100,000 refugees who have fled violence in neighbouring Nigeria, has not been affected by the bad weather.

In 2014, floods have killed dozens of people and affected hundreds of thousands, according to official figures.

The Niger River, the third largest in Africa, has a flood basin of over two million square kilometres (770,000 square miles) which is home to more than 100 million people from Guinea to Nigeria.