AFP - The world urgently needs a climate deal out of a summit in Copenhagen next month to forestall forced migration that is already occurring in Asia and Africa, UN Secretary General Ban Ki-moon said Wednesday.

"We are in a critical period," Ban told delegates at a United Nations conference on the economic impact of immigration.

"Populations will relocate due to more extreme weather, including prolonged droughts, intensive storms, wildfires," he said.

The threat is already visible in the "devastation" wreaked on New Orleans by Hurricane Katrina in 2005, Ban said, adding that perennial floods displace millions in countries like Bangladesh whilst rural areas in Africa are emptying from desertification.

"We need action and agreement in Copenhagen. We will continue to push for the most ambitious targets and the most ambitious agreement possible," he said.

The December 7-18 talks in Copenhagen aim to produce a treaty that will take effect after 2012 when current pledges of the Kyoto Protocol expire, setting medium- and long-term goals for taming global warming.

An EU summit last week agreed that developing nations will need 146 billion dollars (100 billion euros) per year by 2020 to tackle climate change, but failed to nail down how much aid it would provide.

At the immigration forum in Athens, Ban said more people were on the move than ever before, also spurred forward by poverty and war.

"Today, the number of international migrants is greater than at any time in history, with 214 million people living outside their country of birth," he said.

The informal two-day Global Forum on Migration and Development (GFMD) opened in Athens on Wednesday with officials from over 40 international and regional organisations discussing the economic impact of immigration and policy proposals on how to improve the integration of migrants.

"Our goal is to harness the power of migration to reduce poverty and inequality," Ban said.

The annual forum series began in 2006 following a proposal from former Secretary General Kofi Annan.

Greek leftist organisations and migrant and citizens' groups have criticised the event, arguing that governments should be focusing on the treatment of migrants rather than their economic integration.

A protest march against the conference will be held later on Wednesday.