Crunch climate talks began in Copenhagen with leaders and officials urged to achieve "a truly ambitious" deal on tackling global warming.
President of the two-week conference Connie Hedegaard said the world had reached the deadline for achieving a new deal to cut greenhouse gas emissions and deliver finance to support poor countries in the fight against climate change.
"Now is the time to capture the moment and conclude a truly ambitious global deal. This is our chance. If we miss this opportunity, we will not get a better one," she warned.
The UN's chief climate official Yvo de Boer said there was unprecedented political momentum for a deal but warned the conference would only be a success if it "delivers significant and immediate action that begins the day the conference ends".
Developments included South Africa pledging to curb its emissions by 34% below its expected "business as usual" pollution with international help, China suggesting a date for its emissions to peak and the US backing a 10 billion dollar "fast start" fund for poor countries to take immediate action.
And in the US, the Environment Protection Agency has concluded greenhouse gases endanger human life and must be regulated, signalling that President Barack Obama's administration is prepared to tackle global warming without getting Congressional approval.
The closing days of the conference are now set to be attended by 110 world leaders, raising hopes of a political deal on curbing emissions and providing finance to support poor countries in the fight against climate change.
The leaders will join more than 15,000 negotiators, environmental campaigners and journalists descending on the Danish capital in the bid to secure a new deal to stop temperatures rising more than 2C above pre-industrial levels.
Energy and Climate Change Secretary Ed Miliband has warned those who argue climate change is not the result of human actions are "profoundly irresponsible". Mr Miliband said the next two weeks were "crunch time for the planet".
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He said the UK is pushing for the most ambitious, effective and fair deal possible. But he warned if countries fail to take advantage of the current momentum to secure agreement on efforts to tackle rising temperatures it would not get any easier to achieve at a later date.
