About a million people have been affected by torrential rains stretching between West and East Africa since July, with Ghana and Uganda accounting for more than half the tally alone, the UN's humanitarian coordination office (OCHA) said.
Cases of cholera, dysentery and diarrhoea have been reported in northern Ghana, where 260,000 people were affected by floods and the death toll has risen to 32, OCHA spokeswoman Elisabeth Byrs told journalists.
Authorities there have declared a state of emergency and appealed for international help after bridges, homes and crops were destroyed, she added.
All eight districts in Upper East Region have been severely affected by the torrential rains and consequential flooding. Five out of 18 districts in Northern Region are badly affected, meaning that more than 75% of the crop is damaged.
The UN was expecting to launch an appeal for aid for Ghana after a three-day assessment by a team of UN disaster relief experts who reached the stricken areas with the help of Ghanaian authorities on Monday, Byrs said.
"They are expecting huge rainfall in West Africa between September 18 and 24," Byrs told AFP.
The appeal may include neighbouring Burkina Faso, one of the world's poorest nations, she said.
The floods and severe rain in both Uganda and Ghana are being described as the worst seen in each country for at least three decades, Byrs said.
"The floods stretch from the Atlantic coast to the Red Sea more or less," Byrs underlined.
However, the UN's relief coordination office was unable to give a complete picture of the situation across the continent, where more than a dozen countries were last week reported to have been affected to varying degrees.
A team of UN and government officials and civil society organizations are touring the 9 districts to ascertain the extent of damage caused by the rains and the immediate needs of the people.
Government is seeking international humanitarian assistance and food supply from the United Nations and other international bodies.