Lars Loekke Rasmussen, Danish prime minister and the conference chairman, said the meeting was "now at a critical juncture and we have now agreed on how to proceed."
"Now we rely on the willingness of all parties to take that extra step that would enable us to make the deal that is expected of us," he said on Thursday.
The European Union also expressed concern, calling on "all parties to urgently go to the outer limits of their flexibility so that talks can move forward".
Hillary Clinton, the US secretary of state, attempted to put new life into the flagging talks by announcing that Washington would contribute to a $100bn fund to help poor nations with climate change.
But she also accused developing nations of backsliding on pledges to open their emissions control to scrutiny.
"There have been occasions in this past year when all the major economies have committed to transparency," Clinton told a press conference.
"Now that we are trying to define what transparency means and how we would both implement it and observe it, there's a backing away from transparency -- and that to us is something that undermines the whole effort that we're engaged in," she said.
Jonah Hull, Al Jazeera's correspondent in Copenhagen, said Clinton's speech only served to highlight the divisions between rich and poor nations rather than push forward a deal.
"Those who had hoped to see something from the United States were probably disappointed"
Jonah Hull, Al Jazeera's correspondent in Copenhagen
"Those who had hoped to see something from the United States, something more to give the conference in its final stages, a big push, were probably disappointed," he said.
Hull said that this showed the US was "not prepared to move forward unless they [the US] see these big developing countries, the big polluters of the future, doing their bit".
Delegates are hoping talks will be pushed forward by the arrival of world leaders on Thursday, and the US president on Friday, with a Danish official insisting negotiators had "not given up".
Merkel, speaking in Berlin, said the negotiations "do not look promising but I of course hope that the presence of more than 100 heads of state and government can give the necessary impetus to the event."
Yvo de Boer, the United Nations climate chief, has urged delegates from 193 countries to overcome their differences and produce a deal.
Environment ministers are set to meet for final rounds of negotiations on Thursday, aiming to salvage a climate pact from half-finished draft texts and overcome long-running rifts between rich and poor nations over how to split the costs of fighting climate change.
About 120 leaders are expected to attend the summit, including the presidents of Iran, France and Brazil.
Wen Jiabao, the Chinese premier, arrived in Copenhagen on Wednesday evening and Barack Obama, the US president, is due to arrive on Friday.
Denmark, whose prime minister is now chairing the summit, said it was trying to simplify several complex draft negotiating texts to help the leaders attending a final high-level summit on Thursday and Friday agree on a deal.
However, delegates from developing nations have rejected Danish proposals to select small negotiating groups to storm through the draft texts, saying the process had to be fully inclusive.
Among the key sticking points is a long-running rift between developed and developing nations over who should cut emissions, how deep the cuts should be, and how much assistance the rich world should provide to poor countries.
The US and China, the world's top carbon emitters, have also been stuck in a dispute over how they will prove they are sticking to emission-curbing plans.