The torrential rains and floods that have ravaged sub-Saharan Africa from the Atlantic coast to the Indian Ocean are believed to be the worst in three decades and caused by the "La Nina" weather pattern.
An estimated 650,000 people from Ghana to Kenya were in dire need of humanitarian assistance and forecasters were predicting more rain in the days to come.
The International Federation of Red Cross and Red Crescent Societies issued a statement sounding alarm bells over the situation's potential to develop into a major crisis spanning several countries.
"It is evident from the scope of the disaster that a massive aid effort will be needed to help hundreds of thousands of flood victims survive the crisis and rebuild their lives," Niels Scott, operations coordinator for Africa, said in a statement.
Scott said emergency food, shelter and clean water supplies were needed, as well as pre-positioned stocks, flood-proof wells and other measures to prepare for wider flooding.
At least 270 people have died as a result of the floods and waterborne disease, according to an AFP tally from governments and humanitarian aid organisations.
Uganda declared a state of emergency on Thursday across several northern and eastern regions worst-hit by rains and floods that struck an area stretching from east to west Africa since August.
Aid workers say they fear the death toll could rise sharply as diseases threaten the parts of the east African country that are least prepared to deal with them.
The Ugandan move followed in the steps of the west African state of Ghana which declared a state of emergency earlier following exceptional floods in the north of the country.
The Red Cross has launched two initial appeals for relief aid funding so far, one for 290,000 people displaced by floods in Uganda totalling 7.2 million dollars (5.1 million euros) and another 2.1 million dollars for emergency supplies in Ghana and Togo.
In turn, the United Nations' humanitarian agency (OCHA) launched an appeal for 43 million dollars in emergency assistance to flood-affected people in Uganda.
"The Flash Appeal is based on planning for an estimated 50,000 households (300,000 people) being affected by the flooding and requiring humanitarian assistance to varying extents," it said in a statement.
"Amongst the affected population there are 110,000 already vulnerable internally displaced people. Women and children make up the largest percentage of the overall affected population," the statement added.
The Red Cross also underlined major concerns about the impact of the floods in northern Togo.
"The northern Savane region is particularly vulnerable -- a UN 2006 study showed that 62.7 percent of people in the region do not have access to adequate food and one third of children under five years old suffer from malnutrition," Scott said.
"It will be absolutely vital to get emergency food, as well as seeds and planting tools, to these populations," he added.
The hardest hit country was Sudan, where more than 60 people died as a result of the floods in the war-ravaged south.
Neighbouring regions in Uganda, Kenya and Ethiopia were also badly hit and now face the threat of water-borne diseases such as cholera sweeping hundreds of thousands of weak and starving displaced people.
"Most have been rendered food insecure by the loss of their first season harvest and the delay in second season planting," OCHA explained.
"Whereas a two-month 'hunger gap' is the norm, this year it is expected to extend up to 10 months. The next successful harvest cannot be expected before February 2008; if the heavy rainfall continues in the most affected areas, it will be further delayed."
The Netherlands released 11 million euros Friday to aid three African countries most-affected by the heavy downpour.
Two million is earmarked for Uganda, another two for west African nations and the rest to Sudan, said a statement by the the Dutch development ministry.