"It is not yet the end of the rainy season so we do not know just how bad it will get, but we do know the situation is already very serious," said Moustapha Diallo, spokesperson for the International Federation of the Red Cross and Red Crescent Societies (IFRC) West and Central Africa office.
Aid agencies estimate that between 400,000 and 600,000 people have been affected by flooding since the beginning of the rainy season - "affected" meaning they have been displaced, they have suffered loss to homes or belongings or their ability to make an income has been stymied, according to the UN Office for the Coordination of Humanitarian Affairs (OCHA).
The number of displaced across the region has not been calculated but government and aid agency figures indicate it is more than 100,000.
Burkina Faso, Niger, Senegal and Sierra Leone are among the worst-affected, says OCHA, which estimated that over 160 people have died thus far across the region as a result of flooding.
Governments in Burkina Faso and Benin have called on international donors and aid agencies to help them respond to the floods.
"The particular force and extent of the flooding this year is really overwhelming governments' capacity to cope, despite some preparation measures that have been put in place," said Charles Bambara, spokesperson for Oxfam in the region. This is partly due to the sheer force and magnitude of the downpours - Burkina Faso received one quarter of its average annual rainfall in three hours on 1 September according to the national weather centre.
The impact of 2009 flooding to date is not as severe as in 2007, when the region's worst floods in decades killed at least 300 people and affected 800,000, according to OCHA spokesperson Yvon Edoumou.
"If the rains continue in some countries - such as Burkina Faso, Senegal and Sierra Leone - we could find ourselves in serious trouble," he said.
More rain is forecast for Senegal in the coming days, according to regional weather centre ACMAD.
The European Commission's humanitarian aid department, ECHO, has provided US$4.3 million for flood relief in the region, $3 million of which will go to Burkina Faso. The IFRC has allocated $938,000 to respond to flood damage across the region. It is using pre-positioned supplies of plastic sheeting, blankets, water containers and hygiene materials located in Senegal, Benin and Cameroon to speed up its response to within 48 hours of an alert, according to IFRC's Diallo.
Read more on the latest flood response in each country
"Tapping into these supplies has enabled us to move more quickly and respond to people's needs within hours," Diallo told IRIN.
IFRC has distributed relief supplies to flood-affected families in Benin, Burkina Faso, Central African Republic, Gambia, Ghana, Guinea, Liberia, Senegal and Sierra Leone.
Meanwhile some 177,500 people in Burkina Faso, Mauritania and Niger are targeted to receive food aid from the UN World Food Programme. Oxfam, Save the Children and World Vision are among several NGOs sending relief items and water and sanitation experts to assess damage.
Governments and aid agencies say the priority is ensuring affected people have access to clean water and adequate sanitation as well as to essential food and non-food items.
"In the short term having people living next to polluted water and household waste could have dramatic health consequences," Oxfam International's West Africa director, Gerard Steehouwer, said in a 9 September communiqué.
Even in normal conditions, access to water and proper sanitation facilities is low in much of the region. Burkina Faso, Mali, Niger and Sierra Leone rank lowest out of 177 countries evaluated by the UN for access to clean drinking water.
Nearly every year the rainy season brings floods, with conditions exacerbated by a lack of urban planning, poor water and sanitation infrastructure and rapid urban population growth, donors and aid officials say.