In a joint appeal published in the British Medical Journal and The Lancet, they urged governments to act decisively to roll back the threat from global warming.

Scientists have repeatedly warned climate change could affect health in many ways, ranging from malnutrition caused by drought to the risk of cholera from flooding and the spread of mosquito-borne diseases to temperate zones.

''Doctors must take a lead in speaking out. There is a real danger that politicians will be indecisive, especially in such turbulent economic times as these. Should their response be weak, the results for international health could be catastrophic,'' the doctors' letter said.

The December 7 to 18 meeting under the UN Framework Convention on Climate Change aims to set down action for tackling heat-trapping carbon emissions beyond 2012, when the Kyoto Protocol expires.

The letter was signed by the presidents of 18 colleges of physicians or academies of medicine from Australia, the United States, Britain, Canada, Ireland, Thailand, Hong Kong, Malaysia, Ireland, South Africa and Scotland.

Meanwhile, in a challenge to US President Barack Obama before a key international summit in Pittsburgh, the European Union presidency in Brussels said the US must do more to tackle climate change.

Swedish Prime Minister Fredrik Reinfeldt said, ''I hope to speed up the talks all over the climate issue.''

He acknowledged a promise by Japan's incoming prime minister, Yukio Hatoyama, to target 25 per cent cuts in greenhouse emissions, but said he was less impressed by US efforts so far.

The EU prides itself on taking the lead in the battle against climate change, with member states agreeing to make 20 per cent cuts in carbon dioxide emissions by 2020 from 1990 levels. EU leaders are seeking a more ambitious global goal at the Copenhagen talks and are ready to commit to 30 per cent cuts if the rest of the world does likewise to attain the overall goal of restricting global warming to 2 degrees.