United Nations secretary general, Ban Ki-moon, has warned that biofuels could be doing more harm than good with respect to climate change.

During a visit to Brazil he warned that governments must be careful to balance the costs and benefits of developing biofuels as energy sources.

Ki-moon said he was aware of the controversy surrounding biofuels. "Some fear that land currently used to grow food will instead be turned over to fuel," he said, according to the UN news centre. "Others worry that forests will be cut down to make way for biomass plantations. Still more worry about the effects on the environment and biodiversity."

National governments must take the lead in managing their use and ensuring that the benefits outweigh the costs, he said.

The secretary general visited an ethanol plant, which he described as "one of many green technologies that show promise in offsetting global warming and he commended both the Brazilian government and private business enterprises in the South American country for trying to develop clean and renewable sources of energy.

Somewhat controversially he said that; "Brazil is the quiet green giant. It leads the world in renewable energy. It has one of the cleanest energy economies in the world. Brazil is one of the few nations to successfully produce biofuels on a large scale," he said, calling for increased international attention to "what Brazil is achieving."

However, environmentalists argue that it is precisely in Brazil that forests are being cut down to make way for the production of crops for biofuels. Some of the biofuel crops in Brazil are sustainable, but most are not, according to Deepak Rughani of Biofuelwatch. "If biofuel production results in ecosystem loss then it cannot be described as sustainable," he said.

Rughani cited examples of Amazon rainforest destruction to make way for soya crops, which are being grown for biofuels, and of destruction of the Cerrado, the largest savanna in South America, for sugar cane plantations. The cerrado is viewed as 'wasteland' by the Brazilian government, and therefore suitable for sugar cane production. However, according to the Worldwide Fund for Nature, the Cerrado is biologically the richest savanna in the world, home to at least 5% of the world's flora.

Rughani argues that some ecosystems around the world are under severe threat, and that changes to weather patterns have already been observed in the Amazon, for example. Irreversible changes to weather patterns would set off a chain of reactions that could accelerate climate change.

Another senior UN official, Aachim Steiner, head of the United Nations Environment Programme (UNEP), today warned that biofuels may be adding to the problem of climate change rather than alleviating it. He said that increased demand for fuel crops had led to vast swathes of rain forest being destroyed and that international standards should be drawn up to protect them. According to BBC Radio Four, he said that there is a danger that a public backlash might divert investment away from cleaner second generation biofuels.