Negotiators from industrial nations including the United States on Friday said 11th-hour promises are possible and a global warming pact can be reached.
But developing countries complained that pledges so far were nowhere near enough to avoid climate catastrophe, and that world leaders need to participate in the conference to cut a meaningful deal.
"Part of the frustration is that a deal is so close ... all the elements are there," said Kevin Conrad, the delegate from Papua New Guinea. "But it's absolutely conceivable for senior people to come together and spend a week and clean all this up."
The United States was universally seen as the lynchpin to a Copenhagen deal, but it has been unable to present its position or pledge emissions targets because of the slow progress of climate legislation in the Congress. "Everyone else wants to calibrate against" the Americans, Conrad said.
With the U.S. position still unclear, expectations at this week's U.N. talks in Spain shifted toward a political agreement in December in which rich nations would make hard pledges to reduce emissions and to finance aid to help the world's poorest cope with the effects of Earth's rising temperatures.
Under such a deal, nations would agree to stick to their promises while negotiating the details of the treaty, taking as long as a year. If world leaders came to Copenhagen to endorse the deal, those promises would carry more weight, delegates said.
Some 40 world leaders so far were expected to attend the Copenhagen talks, including British Prime Minister Gordon Brown and French President Nicolas Sarkozy. Brazil's President Luiz Inacio Lula da Silva has indicated he may come, and a spokesman for German Chancellor Angela Merkel said she is keeping the date open.
Yvo de Boer, the U.N. official who is shepherding the talks, said negotiators still hoped to achieve a significant deal that would set specific goals.
"Governments can deliver a strong deal in Copenhagen," de Boer said, adding that it would be difficult for developed countries "to wiggle out" of written commitments they make in a Copenhagen deal.
The deal may take the form of consensus decisions, including an overarching statement of long-term objectives, along with a series of supplemental decisions on technology transfers, rewards for halting deforestation, and building infrastructure in poor countries to adapt to global warming, delegates said.
But developing nations were mistrustful of any result that did not hold wealthy nations to legally binding targets, citing a history of broken promises in development aid and famine relief.
The aim of the ongoing negotiations has been to broker a deal building on the 1997 Kyoto Protocol, which expires in 2012. Without a successor accord, carbon emissions will have no international regulation, which would hinder the ability of industry to factor in the price of carbon and plan future business.
While some countries, such as Germany and Britain, are meeting their Kyoto emission-reduction targets, others have not. Canada's emissions, for example, grew by more than 25 percent from 1990 to 2007, U.N. figures show, although it committed under Kyoto to reduce them 6 percent from 1990 levels by 2012. Japan's grew 9 percent during that period, compared with a target of minus 6 percent.
De Boer was looking to the United States to announce a clear emissions target for 2020, saying "a number from the president of the United States would have huge weight."
"The United States is interested in the strongest possible agreement we can get from this process," said Jonathan Pershing, the chief U.S. delegate to the U.N. talks. He showed impatience with developing nations for wanting to hold rich nations to legally enforceable targets while arguing they should be exempt from them.
"We are looking for parallelism. We are not looking for imbalance," he said.
He declined to say whether the U.S. will be ready to submit a target for the Copenhagen accord. He noted that President Barack Obama has the authority to make a commitment without congressional approval, "but a decision on whether or not we will do it has not yet been made."
U.N. scientists say rich countries must cut carbon emissions by 25 to 40 percent from 1990 levels by 2020 to prevent Earth's temperatures from rising 2 degrees Celsius (3.8 F) above its average temperature before the industrial era began 150 years ago. Anything higher could trigger climate catastrophe.
So far, reduction pledges total 11-15 percent.
But those pledges could also be seen as opening bids that could be upped as negotiations grow more intense. "Somebody's got to start creating that dynamic. Either it happens or doesn't," said Conrad, the Papua New Guinea delegate.
The wider issue of ending the Copenhagen conference without a legally binding agreement disappointed developing nations already suffering severe droughts, floods and other disasters blamed on rising temperatures.
"We totally reject it ... it would be a mockery," said Sudanese delegate Lumumba Di-Aping, representing the bloc of developing nations.
In the meantime, world leaders will be discussing the climate issue at major meetings planned before the Dec. 7-18 Copenhagen talks, including an upcoming meeting by the 17 nations in the Major Economies Forum.
Developing nations one by one urged negotiators not to give up on a legal pact at Copenhagen.
"We don't share view that it is no longer possible. If it were no longer possible, we would rather pack up and go home," said the Indian delegation chief, Shyam Saran. He said any agreement signed by all 192 nations at Copenhagen could still be binding.
South Africa's chief negotiator Alf Wills warned other nations against promoting a watered-down text, saying "we will not accept a weak, green-wash outcome."
The chairman of the 43-member Alliance of Small Island States also urged world leaders to seek a legally binding pact. "Weak political declarations are not the solution," Grenada Ambassador Dessima Williams said.
The European Union, led by Sweden, said it was pushing for the most ambitious deal possible. "We are going to change the fundamentals of industrial civilization, so it's no wonder there is a lot of activity going on in a negotiation like this," Swedish delegate Anders Turresson said.