World leaders worked through the early hours to try and beat a Friday deadline for a deal on cutting emissions and helping poor countries cope with the costly impact of global warming.
After days of stalemate, the United States revived the 193-nation talks on Thursday by backing a $100 billion climate fund to help poor nations adapt their economies and tackle threats such as failing crops and dwindling water supplies.
A group of about 25 influential world leaders had constructive talks overnight on how to unblock the climate negotiations, Danish Prime Minister Lars Lokke Rasmussen, who hosted the talks, said on Friday.
We had a very fruitful, constructive dialogue, Rasmussen told reporters.
Many leaders mentioned risks of failure ahead of the final push, which started with a gala dinner for about 120 world leaders at Christiansborg Palace in Copenhagen, hosted by Denmark's Queen Margrethe.
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Time is against us, let's stop posturing, said French President Nicolas Sarkozy, one of scores of leaders who addressed the talks on Thursday. A failure in Copenhagen would be a catastrophe for each and every one of us.
Police said 28 people were detained in connection with a Greenpeace protest near the palace, including three who evaded security to slip inside.
After arriving in a motorcade ahead of Secretary of State Hillary Clinton, the activists walked straight up the red carpet carrying signs reading:
Politicians talk, leaders Act.
U.S. President Barack Obama will arrive on Friday and is expected to face pressure to pledge deeper emissions cuts from the world's number two emitter of greenhouse gases behind China.
I really expect them to announce something more, European Commission President Jose Manuel Barroso told reporters.
President Obama is not coming just to reiterate what is in their draft legislation, he said, referring a climate bill that has yet win U.S. Senate approval.
Obama will meet Chinese Premier Wen Jiabao on the sidelines of the Dec 7-18 talks, the largest ever climate summit.
Officials said the United States was making progress with China on outstanding issues but could not say whether a deal would result after Obama arrived.
One U.S. official said there was progress on monitoring, reporting and verification requirements by China and other big developing countries on their emissions curbs. China has resisted such requirements.
The United States had helped the mood earlier by promising to back a $100 billion a year fund for poor nations from 2020.
Such funds would be more than all current aid flows to poor nations, a U.N. official said, and in line with demands put forward for African nations. That's very encouraging, U.N. Secretary-General Ban Ki-moon said of the U.S. pledge.
A U.S. official said Obama was unlikely to be more specific about U.S. funding commitments.
Accord on finance is one part of a puzzle that also includes a host of other measures, such as saving rainforests, boosting carbon markets and stiffening global carbon emissions curbs.
If each and everyone does a little bit more then we can do this, German Chancellor Angela Merkel said. She said the European Union was willing to do more but would not act alone.
But any deal will have to be agreed by unanimity. Some small island states and African nations -- most vulnerable to climate change -- say they will not agree a weak deal.
We are talking about the survival of our nation, Prime Minister Apisai Ielemia of the Pacific island state of Tuvalu said of the talks that began two years ago in Bali, Indonesia.
The draft texts of the negotiations include possible goals such as halving world greenhouse gas emissions by 2050 or obliging developed nations to cut their emissions by between 25 and 40 percent by 2020.
We are moving out of the valley of death. We are beginning to see the outlines of a compromise, helped by the U.S. offer on finance, said Kim Carstensen, head of the WWF environmental group's global climate initiative.
Earlier on Thursday, prospects for a strong U.N. climate pact seemed remote as nations blamed leading emitters China and the United States for deadlock on carbon cuts. But ministers and leaders urged fresh urgency.
Copenhagen is too important to fail, China's climate change ambassador Yu Qingtai said.
(With extra reporting by Alister Doyle, Anna Ringstrom, John Acher, Jeff Mason, Richard Cowan, Emma Graham-Harrison, Krittivas Mukherjee and Karin Jensen; Writing by Pete Harrison and Gerard Wynn; Editing by Ralph Gowling)