COPENHAGEN - With time running out on the stalled Copenhagen climate negotiations, Secretary of State Hillary Rodham Clinton gave new hope that an agreement might still be reached when she announced Thursday that the United States would help raise $100 billion a year by 2020 to help poor nations combat climate change.

The talks are scheduled to end Friday, when President Obama and more than 100 other heads of state are due to arrive.

Mrs. Clinton's announcement signaled the first time the Obama administration had made a commitment to a medium-term financing effort, even though she did not specify the American contribution to this fund. She also cautioned that the United States' participation was contingent on reaching a firm agreement this week, one that would require a commitment from China about greater transparency in its emissions reporting.

"A hundred billion can have tangible effects," Mrs. Clinton said. "We actually think $100 billion is appropriate, usable and will be effective."

The $100 billion figure is in line with estimates by Britain and the European Union of the needed contributions, although the amount is at the low end of the range that European countries have suggested.

Shortly after Ms. Clinton's announcement, Yvo de Boer, the head of the United Nations climate office, welcomed the decision by the United States to support the fund and said he saw it as a sign that negotiations were making some progress.

"Hold tight," Mr. de Boer said. "Mind the doors. The cable car is moving again."

But Mr. de Boer also sounded some cautious notes, saying that it was important to wait and see "if that sum is adequate" in the view of other nations, and he called on the Americans to put a specific amount of money on the table for the fund. Mr. de Boer also underlined that structures would need to be drawn up to control the disbursement and management of the money.

Mrs. Clinton said the money would be a mix of public and private funds, including "alternative sources of finance," which she did not specify. Nor did she say what the American share of the fund would be, although typically in such multilateral financial efforts the United States contributes about 20 percent. She said the money should chiefly flow to the poorest and most vulnerable nations and should contain a sizable amount to slow deforestation, which contributes to carbon dioxide concentrations in the atmosphere.

The British government released a statement shortly commending Mrs. Clinton's announcement.

"It's an important development and very welcome to have the United States on the same page as the U.K. and the E.U. in terms of long term climate finance," the statement read.

The announcement from the United States seemed to shift the pessimistic tone of the talks in which China had signaled overnight that it saw virtually no possibility that the nearly 200 nations gathered would find agreement by Friday.

A participant in the talks said that China would agree only to a brief political declaration that left unresolved virtually all the major issues.

The conference has been deadlocked over emissions cuts by, and financing for, developing nations, including China, who say they will bear the brunt of a planetary problem they did little to create. Leaders had hoped to conclude an interim agreement on the major issues that would have "immediate operational effect." The Chinese, it appears, are not willing to go that far at this meeting.

Whether the Chinese position represents political brinkmanship as senior ministers and heads of state begin arriving in Copenhagen for the final 48 hours of negotiations, or a genuine signal that Chinese officials are not inclined to settle the wide differences separating it and developed nations, was unclear on Thursday morning.

Until Mrs. Clinton's announcement on Thursday, the world's two richest blocs, the European Union and the United States, had been slow to put pledges on the table for long-term financing, which under most estimates would require them to pay billions of dollars each year by 2020. Last Friday, European Union leaders agreed on short-term financing totaling $10.5 billion over the next three years to help poor countries begin tackling the effects of global warming. But the bloc has so far failed to agree how much they would give in long-term financing. European experts have recommended that the fund should total about $150 billion annually by the end of the next decade.

Until Mrs. Clinton's announcement, the continued bickering among delegations had seemed to be making the likelihood of a significant breakthrough increasingly slim.

On Wednesday night, Mr. de Boer seemed skeptical. but warned that "the next 24 hours are absolutely crucial and need to be used productively."

The continued deadlock is due in large measure to delays and diversions created by a group of poor and emerging nations intent on making their dissatisfaction clear. The Group of 77, as it is called, has raised repeated objections to what its members see as the economic and environmental tyranny of the industrial world.

On Monday, African nations briefly brought the climate talks to a standstill. China, by far the largest economic power in the group, has dragged its feet throughout the week by raising one technical objection after another to the basic negotiating text. And on Wednesday night, the group refused to take part in negotiations that conference organizers had hoped would produce a definitive negotiating text by Thursday morning. Instead, many Group of 77 leaders spent the day hurling accusations at wealthier countries.

President Obama and other world leaders have said that the Copenhagen meetings are unlikely to produce a binding treaty; some sort of interim political agreement is far more likely, they said. But few appreciated the depth of anger in the developing world and the height of grandstanding that would consume so much of the conference's time. Now it is hard to find someone who confidently predicts even that much success.

The Group of 77 is a group in name only. Made up of 130 countries, it represents tiny island nations like Vanuatu and advanced middle-income states like Argentina. Many developing nations have united under the group's auspices because they can take advantage of the far greater negotiating power and resources of countries like China and Brazil. Many small countries have neither a big enough delegation nor the organizational structure to negotiate effectively on their own.

China has been a natural godfather to many of the Group of 77 countries because its government has extensive investments in Africa and Latin America, often involving lucrative deals to bring oil and minerals home.

The coalition is united on a few central issues. They include making sure that industrialized countries keep the emissions reductions pledges they made as part of the 1997 Kyoto Protocol and that the Copenhagen conference produces enough money for poorer countries to adapt to climate change, said Maria Fernanda Espinosa, Ecuador's minister of cultural and ecological patrimony.

But the group is neither a tight negotiating unit, nor particularly well organized. While larger countries like Brazil and China have well-appointed headquarters in one part of the Bella Center, where the negotiations are being held, the Group of 77 office itself is made up of two spartan rooms equipped with two computers, where some delegates from the poorest African nations sat Wednesday morning drinking soda and nibbling biscuits.

"The G-77 is an incredibly diverse group," said Michael A. Levi, a climate change specialist at the Council on Foreign Relations who is attending the Copenhagen meeting. "Its richest countries are 50 times as wealthy on a per-capita basis as its poorest ones. All of this makes a common yet constructive position very difficult. The easiest thing to agree on is to obstruct action."

The cost of such obstruction is growing higher by the day. On Thursday and Friday, ministers and heads of government are expected to fashion a complex political agreement encompassing a host of issues that have divided them for years. Seldom, if ever, have national leaders engaged in negotiations as complex - and as poorly prepared - as these.

The strain is showing both inside the Bella Center and outside. On Wednesday, hundreds of demonstrators tried to storm the hall, but were pushed back by truncheon-wielding riot police officers who made 260 arrests. Inside, numerous groups staged demonstrations, sit-ins and noisy disruptions of public sessions.

Mr. de Boer, the United Nations official in charge of the conference, said that he was concerned about the safety of the arriving leaders and the rest of the participants. "The incidents that have taken place today inside the conference center test my courage to continue in this way," he said, suggesting he would sharply limit access to the hall for the final two days.

By John M. Broder and Tom Zeller Jr., The New York Times, December 17, 2009