The official said the Danish hosts of the U.N. conference had not given up though it appeared unlikely that their ambitious plan for the conference would be fulfilled.
"As it looks now, we will not get the deal that we had hoped for," said the official, who is not authorized to speak publicly about the talks and asked not to be named.
Denmark started the two-week U.N. conference - the largest and most important meeting on climate change in history - hoping to crafting a comprehensive framework to curb the greenhouse gas emissions that cause global warming and provide funds for poor countries to deal with climate change.
They wanted to get agreement on all the main elements so that a treaty could be signed next year.
But so far the talks have been marked by sharp disagreements between China and the United States - the world's top carbon polluters - and a yawning chasm between rich and poor nations over what should be done.
Addressing the conference Thursday, Secretary of State Hillary Rodham Clinton symbolically chose to try and bridge the gap by quoting what she said was a Chinese proverb.
"When you are in a common boat, you have to cross the river peacefully together," Clinton told the conference attendees. "We are in this boat together."
Clinton said she believed there was still "a way forward" for the negotiations, and she announced that, if an agreement was reached, the U.S. would "work with other countries toward a goal of jointly mobilizing $100 billion per year" to help address the needs of poor and developing countries coping with climate change.
Asked whether President Obama might chose not to attend the last day of negotiations as he's scheduled to do, should a still deal look unlikely, Clinton said he was, "planning to come tomorrow. Obviously, we hope that there will be something to come for."
Mr. Obama is scheduled to arrive in Copenhagen Thursday night and join the talks on Friday.
Still unresolved are the questions of emissions targets for industrial countries, billions of dollars a year in funding for poor countries to contend with global warming, and verifying the actions of emerging powers like China and India to ensure that promises to reduce greenhouse gas emissions are kept.
CBS News correspondent Mark Phillips reports that the prospects for a deal now range from dismal to grim. The sticking points are basic:
• What should be the target for the reduction of greenhouse gasses by both the developed and developing world.
• How much will the rich countries give the poor to help them adapt to a warming world.
• How will any agreement be verified to make sure countries live up to their promisses.
Clinton put an emphasis on the need for any agreement to includ verification measures by repeatedly calling for "transparency".
If there is still slim hope for an agreement, Phillips says it was expressed by the leader of one of the countries first affected by global warming; that politicians who are now arriving at the conference can do what their underlings could not.
"I would ask all leaders to lead. I would like to tell them that this is not the time to go with the pack but this is the time to go out and convince your countrymen of the right thing," said President Mohamed Nasheed of the Maldives.
European officials also called for a breakthrough in the final stretch of the conference, which is set to end Friday.
"We are in a crisis of the negotiation. We have to overcome the blockage in the discussion and negotiations," German Environment Minister Norbert Roettgen said. "We still have time: 36 or 48 hours. We need the political will to overcome this."
German Chancellor Angela Merkel called on the U.S. Thursday, just before leaving Berlin for the conference, to pledge deeper emissions cuts.
"I believe this Copenhagen conference is the primary touchstone for whether we will succeed in setting a new path of global development, of sustainability," Merkel told lawmakers.
Merkel called on all industrialized nations to make deeper cuts in emissions of carbon dioxide and other global-warming gasses, but singled out the United States.
"I have to be honest, an offer by the United States to cut only 4 percent, from 1990 levels, is not ambitious enough," Merkel said.
The EU has pledged a 20 percent cut that could increase to a 30 percent cut if other developed nations also make far-reaching pledges.
At the same time, she urged developing countries such as China and India to rein in their emissions growth, warning that without their commitments, there would be no way to reach a binding international agreement.
China is the world's largest carbon polluter, followed by the United States.
British Prime Minister Gordon Brown said he believed the conference would still reach an agreement.
"We can, by working together over the next 48 hours, reach agreement that will help the planet move forward for generations to come," he said.
Leaders arriving Thursday included Iran's Mahmoud Ahmadinejad, Germany's Angela Merkel, France's Nicolas Sarkozy, among many others.