* African ministers in common front ahead of December talks

* Ministers seek access to funds to combat climate change

NAIROBI, May 29 (Reuters) - The challenges facing Africa to fight climate change are enormous and costs are huge though hard to quantify, South Africa's environment minister said on Friday.

Minister Buyelwa Sonjica said her peers from more than 30 African countries, meeting in Nairobi, had agreed a joint position on climate change, to be presented at negotiations in Copenhagen this December.

"Increased support to Africa should be based on priorities which include adaptation, capacity building, financing and technology development and transfer," she told a news conference.

The world's poorest continent is expected to be hardest hit by climate change, despite having the lowest emissions of greenhouse gases.

According to the United Nations, between 250 and 750 million people in Africa will face water shortages by 2020, while in some African countries, yields from rain-fed agriculture could be reduced by up to 50 percent by 2020.

"Most of the work remains to be done, particularly with the cost of adaptation in Africa estimated between $1 billion to $50 billion per year," Sonjica said.

Sonjica, who is the president of the African Ministerial Conference on the Environment, urged rich countries not to attach conditions on assistance given to developing countries.

"African governments must commit certain amounts from their budget, but you cannot commit what you don't have. I get a sense that there is a push for us to over-commit," she said.

COSTS

The United Nations has estimated the costs of adapting to rising seas in Africa could amount to at least 5-to-10 percent of gross domestic product towards the end of this century.

It also projects an increase of 5-to-8 percent of arid and semi-arid lands in Africa by 2080.

Africa advocates expansion of categories so that countries can benefit from carbon credits, and other international incentives to include agriculture and forest management.

Yvo de Boer, head of the U.N. Climate Change Secretariat, said bigger countries such as China and Korea had won more funds than Africa from rich nations to help cut greenhouse gases.

Africa has received relatively little aid because the projects available in the continent were much smaller, he told a news conference.

"Because many economies in Africa are very small, not a lot can be done to reduce emissions gases, so reducing emissions in Africa is not as big an opportunity as it is in Asian countries," he said.