Annan said he wanted the impact of climate change on refugee flows and humanitarian strife around the world to be a first priority of a new forum he is launching on October 17.

"This is perhaps the single most important humanitarian challenge of years to come," Annan told journalists in Geneva.

"Climate change is already taking place and affecting the lives of thousands of communities," he said.

The former UN chief highlighted the floods in recent weeks across a swathe of sub-Saharan Africa, the plight of some 10 million East Africans hit by floods or drought in recent years, and an estimated 20 million in South Asia affected by heavy monsoon flooding this year.

The Global Humanitarian Forum being set up by the former UN Secretary General, with Swiss government funding, aims to plug a gap in international disaster relief and prevention by bringing together governments, aid agencies, the military, the business world and academics.

"This forum will serve as a catalyst and will build ties... that's something which isn't being done at the moment," said Swiss Foreign Minister Micheline Calmy-Rey.

Annan, who left the top UN post in January, said he wanted to "eliminate these rigid divisions" between different actors which he said stifled prevention measures or hampered the delivery of relief aid.

"We have too much focus on reaction, we sit and wait for things to happen. We have to have a change in culture," Annan said.

The new Geneva-based forum's foundation board is due to meet for the first time next month.

Several of Africa's poorest countries warned on Friday that they were in dire need of assistance due to severe floods, extending from Ghana in the west to Sudan in the east, that have left more than 200 people dead.