Severe flooding across Africa has wrecked hundreds of thousands of homes and left many people vulnerable to water-borne diseases, officials say.
Scores of people have died and much of the continent's most fertile farmland has been washed away in what is being described as a humanitarian disaster.
The UN said more rain was expected and warned that the need for food, shelter and medicine was urgent.
Some 17 countries have been affected in West, Central and East Africa.
UN spokeswoman Elisabeth Byrs said: "The rains are set to continue and we are really concerned because a lot of people are homeless and infectious diseases could emerge.
"Some of the poorest countries, like Mali, Burkina Faso and Niger - the poorest nation in the world - are badly affected."
The UN said the floods could lead to locust infestations and outbreaks of diseases such as cholera and dysentery.
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 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.
"In West Africa, the level of awareness is not the same, and the response capacity [is] really different."
Ghana has been hit badly by the flooding, with three northern regions being declared an official disaster zone after whole towns and villages were submerged.
Information Minister Oboshie-Sai Cofie said: "It is a humanitarian disaster. People have nowhere to go. Some of them are just hanging out there waiting for help to come."
She said the Ghanaian government had received considerable aid and hoped the situation would improve.
French military helicopters were helping relief efforts in nearby Ivory Coast, while officials in Togo were dealing with more than 60,000 displace people and a wrecked infrastructure.
In East Africa, the brunt of the torrential rain was felt in Uganda, Ethiopia and Sudan.
The UN relief co-ordinator in the Sudanese capital Khartoum, John Clarke, told the BBC more than 250,000 had been left homeless there.
In Uganda, where similar numbers have been affected, some villages have been completely cut off by the floods.
Joseph Amongin, a local chief in Magoro town which has been surrounded by flood water, said houses were collapsing and roads were impassable.
The BBC's Sarah Grainger, in Magoro, says people were using dug-out canoes to move around and many of the 7,000 inhabitants have lost their crops.
Musa Ecweru, the country's disaster preparedness minister, said the situation was "getting worse by the hour".
"Access to some communities is almost impossible. We will need boats and helicopters to deliver emergency interventions," he said.
The airlifting of food and medical supplies to affected areas is expected to start on Monday or Tuesday. The UN has diverted a helicopter from Darfur in Sudan to help with the effort.
In Ethiopia, deaths have been reported and a massive food aid programme has been set up after flooding hit almost 200,000 people.
And Rwandan officials reported 15 deaths and 500 homes washed away since Wednesday.