Severe flooding across east, central and west Africa has destroyed hundreds of thousands of homes, killing at least 250 people, and washing away much of the continent's most fertile farmland.
More rain is expected and aid agencies are warning that the need for food, shelter and medicine in the affected regions is urgent.
By last night at least 15 countries across Africa were thought to be affected by the flooding, from Senegal in the west to Kenya in the east. West Africa has suffered most with deaths recorded in Burkina Faso, Togo, Mali and Niger.
Ghana has been hardest hit, with an estimated 400,000 affected, many of whom are now homeless. At least 20 people have died and the floods have also washed away much of the region's crops and livestock. The country's Information Minister, Oboshie-Sai Cofie, said: "It is a humanitarian disaster. People have nowhere to go."
There are fears that the worst is yet to come as those affected by flooding fall prey to water- borne diseases such as cholera. United Nations spokeswoman for humanitarian affairs, Stephanie Bunker, said: "The situation is bad and more rain is likely."
The UN's situation report into the flooding last week said more than half a million people in 12 countries have been affected. The new situation report, due out later this week, is expected to put the number affected at more than one million in at least 15 countries. "The numbers will probably rise as the assessment continues," said Ms Bunker.
Three governments in west Africa have already appealed to the UN for assistance – Ghana, Burkina Faso and Mauritania. Many parts of east Africa regularly flood at this time of year and some preparations had been put in place. Even so more than 250,000 have been left homeless in the Sudanese capital, Khartoum, while nearly 200,000 are in need of assistance in Ethiopia.
In Ethiopia's Afar region the river Awash flooded causing a dam to collapse. Around 4,500 people are still stranded, surrounded by water.
More than 170 schools are reported to be under water in northern Uganda where at least 150,000 people have been displaced and 21 people have died. In many parts of northern Uganda people have only just begun returning to their homes after more than 20 years of fighting between the government forces and the rebel Lords' Resistance Army.
Yvon Cuppens, head of mission in Uganda for Medecins Sans Frontieres visited northeastern Uganda three weeks ago when the flooding began. "A lot of people only planted for the first time this year," she said. "Now most of their fields are under water. During the day you see people going back [to their fields] to try to save some of their crops. I saw small piles of cassava drying on the side of the roads everywhere."
In Rwanda, 15 people have been reported dead and 500 homes washed away, while in Mali at least five bridges have collapsed and hundreds of homes have been destroyed.
Some parts of Africa have been struggling with floods for several months. The UN and the Sudanese government launched a £10m appeal last month to cope with floods described by Sudanese officials as the worst in living memory.
Although flooding is common throughout Africa aid workers said this year's rainfall had been particularly bad. Beatrice Spadacini, a spokeswoman for Care, said the organisation was on "high alert" throughout Africa. "The immediate responses are likely to focus on food aid, provision of safe water, shelter for displaced people and non-food items."
Ms Bunker added: "The situation is much worse than last year. This time last year there were just 65,000 people affected by floods."