US President Barack Obama has arrived for the leaders' session
Leaders have gathered for the final scheduled day of the UN climate summit, amid uncertainty over the shape of any eventual deal.
A draft political agreement drawn up by a small group of countries including the UK, US and Australia was rejected during overnight discussions.
Delegates described the situation as "confusing" and "desperate".
US President Barack Obama told the conference he had come to the summit "not to talk but to act".
Addressing delegates on Friday, he said: "While the science of climate change is not in doubt, I think our ability to take collective action is in doubt right now, and it hangs in the balance."
As well as the leaders' session, talks are scheduled on texts that sources say remain full of fundamental divisions.
One developing country negotiator told BBC News that the rejected draft political accord had arrived "as if from God".
BBC environment correspondent Richard Black said some developing countries have repeatedly complained during the two weeks here of high-handed treatment by the Danish hosts and the West in general.
The summit as of this morning is a summit in crisis
Achim Steiner UN Environment Programme
"It is very confusing, and developing countries are very disappointed because they've invested a lot of time in the documents they're negotiating here - the Kyoto Protocol discussions have been going on for four years," said Martin Khor, executive director of the South Centre, a Geneva-based think-tank.
"Now this other thing comes to undermine it, and people feel their time has been wasted," he told BBC News.
Achim Steiner, director of the UN Environment Programme (UNEP), told BBC News: "Only the heads of state can bring this summit to a successful conclusion."
He added: "But the summit as of this morning is a summit in crisis."
US-LED COPENHAGEN DEAL
No reference to legally binding agreement
Recognises the need to limit global temperatures rising no more than 2C above pre-industrial levels
Developed countries to "set a goal of mobilising jointly $100bn a year by 2020 to address the needs of developing
On transparency: Emerging nations monitor own efforts and report to UN every two years. Some international checks
No detailed framework on carbon markets - "various approaches" will be pursued
There were reportedly a number of sticking points over the draft agreement. One of them appears to have been the absence of a commitment to a legally binding treaty, which many developing countries have been insisting on.
"This declaration has all the meaning and authority of a bus ticket, and certainly won't save the planet," said Joss Garman of campaign group Greenpeace.
"The leaders have just hours left to end this farce but they still can. If they don't step up now this conference will forever be branded the shame of Copenhagen."
President Obama is due to meet individual leaders including the Chinese premier, Wen Jiabao. Correspondents say it is the US and China - the world's two largest carbon emitters - that hold the key to striking an agreement.
Both countries have indicated they might make concessions. China signalled it might allow some monitoring of emission curbs while the US said it would commit money for developing countries.
Addressing the meeting on Friday, UN Secretary-General Ban-ki Moon urged delegations to "seal a deal", adding: "It will be your legacy."
Despite many expressions of concern about projections of climate change, finance has emerged as an issue more likely to make or break a deal than emission pledges, the BBC's Richard Black reports from Copenhagen.
US Secretary of State Hillary Clinton said the US was prepared to help establish funding of $100bn a year for developing countries if a deal emerged that met US requirements.
The key US demand is "transparency" from China, seen as a must if the US Senate is to pass legislation controlling emissions.
COPENHAGEN CLIMATE SUMMIT
Delegates from 193 nations are in Copenhagen to negotiate an agreement on curbing greenhouse gas emissions, in order to prevent dangerous climate change
Developing nations want rich nations to cut emissions by at least 25% by 2020 - rich nations are reluctant to go so far and want developing countries to curb emissions too
The US will not accept legally binding emissions cuts unless China does the same. China has been vague on allowing international scrutiny of its emission cuts
Ongoing disagreement on how funds to mitigate and adapt to climate change will be provided. Poor nations want direct aid, while the West favours schemes like carbon trading
While Beijing has been hostile to this notion, Chinese Vice Foreign Minister He Yafei said China was ready to engage in "dialogue and co-operation that is not intrusive, that does not infringe on China's sovereignty".
There was no immediate reaction from the US delegation to the Chinese offer but, an Associated Press correspondent reported that it went a long way toward meeting American demands.
Earlier, Indonesia's President Susilo Bambang Yudhoyono called for all states, both developed and developing, to be flexible about verification.
He indicated the possibility of setting up an international mechanism for monitoring emission cuts.
The draft declaration is reportedly set to mention a cap of 2C but a document prepared by the UN climate convention secretariat, which was leaked earlier, confirms that current pledges on cutting greenhouse gas emissions are almost certainly not enough to keep the rise in the global average temperature within that level.
The analysis says that to achieve that goal, global emissions should be kept at or below 44 gigatonnes (Gt) of carbon dioxide equivalent (CO2e) in 2020.
But if enacted, the current maximum pledges from developed countries would leave emissions 1.9Gt above that figure; minimum pledges would mean missing the target by 4.2Gt.
Unless this gap is closed, it says - for example by developed nations raising their current overall offer to a cut of 30% from 1990 levels by 2020 - global emissions will "remain on an unsustainable pathway that could lead to concentrations equal or above 550ppm, with the related temperature rise around 3C".
"The stark message for world leaders at Copenhagen is that the proposals on the table - especially from industrialised countries - fall far short of what the world needs," said Keith Allott, head of climate change for WWF in Britain.
In another development, President Obama may reach an agreement in principle on nuclear arms reduction with his Russian counterpart, Dmitry Medvedev, when they meet in Copenhagen on Friday, a senior US official said.
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