The biggest climate talks in history opened on Monday with a stark UN warning of the risk of desertification and rising seas and an assurance by hosts Denmark that a deal to combat climate change was 'within reach.'

Politicians and scientists urged the December 7-18 talks, attended by 15,000 delegates from about 190 nations, to agree immediate action to curb greenhouse gases and come up with billions of dollars in aid and technology to help the poor.

Danish Prime Minister Lars Lokke Rasmussen said 110 world leaders, including US President Barack Obama, were signed up to attend a summit at the end of the meeting.

'A deal is within our reach,' Rasmussen said. But the talks will have to overcome deep distrust between rich and poor nations on sharing out the burden of curbing emissions, mainly from burning fossil fuels.

The presence of so many world leaders meant 'an opportunity the world cannot afford to miss,' he said of the talks, aimed at agreeing a pact to replace the existing UN Kyoto Protocol that runs to 2012.

'The clock has ticked down to zero. After two years of negotiations the time has come to deliver,' said Yvo de Boer, the head of the UN Climate Change Secretariat.

Rajendra Pachauri, the head of the UN panel of climate scientists, said action was needed to avoid more intense cyclones, heatwaves, floods, and possible loss of the Greenland ice sheet, which could mean a sea level rise of 7 metres over centuries.

He said that even a widely accepted goal of limiting global warming to a maximum of 2 degrees Celsius over pre-industrial times could still bring an increase in sea levels that 'could submerge several small island states and Bangladesh.'

'The evidence is now overwhelming that the world would benefit greatly from early action, and that delay would only lead to costs in economic and human terms that would become progressively high,' he said.

He also defended the findings by his panel after leaked emails from a British university last month led sceptics to say that researchers had conspired to exaggerate the evidence.

He said there were rigorous checks on all research. 'The internal consistency from multiple lines of evidence strongly supports the work of the scientific community, including those individuals singled out in these email exchanges,' he said.

Developing nations urged the rich to lead. Papua New Guinea delegate Kevin Conrad said that some small island states 'may soon disappear for ever' without action. African nations said they faced worsening risks of desertification or floods.

Outside the conference centre, delegates walked past a slowly melting ice sculpture of a mermaid, modelled on the Danish fairy tale of 'The Little Mermaid,' as a call for action.

Other activists asked delegates arriving at the conference centre, with a large wind turbine nearby, to go through a green gateway marked 'Vote Earth' or a red one marked 'Global Warming.' They told off anyone choosing red.

The attendance of the leaders and pledges to curb emissions by all the top emitters - led by China, the United States, Russia and India -- have raised hopes for an accord after sluggish progress in negotiations over the past two years.

South Africa added new impetus, saying on Sunday it would cut its carbon emissions to 34 per cent below expected levels by 2020, if rich countries gave financial and technological help.

World leaders did not attend when environment ministers agreed the Kyoto Protocol in 1997.

British Prime Minister Gordon Brown, writing in the Guardian newspaper on Monday, said: 'The British government is absolutely clear about what we must achieve. Our aim is a comprehensive and global agreement that is then converted to an internationally legally binding treaty in no more than six months.'

He added: 'If by the end of next week we have not got an ambitious agreement, it will be an indictment of our generation that our children will not forgive.'

Some 56 newspapers from 45 countries including The Guardian, Le Monde, El Pais and Toronto Star on Monday published a joint editorial urging world leaders to take decisive action.

'Humanity faces a profound emergency. Unless we combine to take decisive action, climate change will ravage our planet,' it said.-Reuters