More than a million people across a swathe of 17 African countries are suffering the effects of severe floods.
Latest reports say 250 people have died and hundreds of thousands of homes have been washed away on some of the continent's most fertile land.
The UN now fears the floods could lead to major outbreaks of water-borne diseases such as cholera and dysentery.
OCHA spokeswoman Elisabeth Byrs said: "The rains are set to continue and we are really concerned".
Ghana has set up an inter-ministerial task force to oversee both immediate relief and long-term reconstruction of the three northern regions devastated by the floods.
Eighteen people have died there and thousands made homeless.
Also badly affected are Burkina Faso (33 dead), Togo (20 dead), Mali (15 dead), and Niger (12 dead).
French military helicopters were helping relief efforts in nearby Ivory Coast, while officials in Togo were dealing with more than 60,000 displaced people and a wrecked infrastructure.
Countries in East Africa regularly flood at this time of year, but West African nations are much less able to deal with the deluge, the World Food Programme says.
In East Africa, the brunt of the torrential rain was felt in Uganda, Ethiopia and Sudan.
Hundreds of thousands of people lost their homes across southern Sudan where the death toll has reached 64.
The UN relief co-ordinator in the Sudanese capital, Khartoum, John Clarke, told the BBC more than 250,000 had been left homeless there.
The UN diverted a helicopter from Darfur in Sudan to airlift food and medical supplies to affected areas.
Meanwhile, Rwandan officials reported 15 deaths and 500 homes washed away since Wednesday in flooding they blame on deforestation.
In its neighbour Uganda, some 21 deaths are being reported from flooding with 150,000 people displaced and more than 170 schools in the northeast are underwater.
The swampy Budalangi region of Kenya floods most years - but people were still caught out by the speed of the rising waters and at least 12 died.
An estimated 200,000 people have been affected in Ethiopia where at least 17 people died and a massive food aid programme has been set up.
"In Kenya or Ethiopia these countries are facing floods every year and year after year, they have set up some contingency plans," the WFP's Pierre Lucas told the BBC.