The $100 billion figure was the first number offered by Washington in discussions here over long-term climate aid, but it falls short of what some experts suggest will be needed. Yvo de Boer, U.N. climate chief, said talks would now focus on the "adequacy" of that target.
Clinton made the offer contingent on the conference's reaching a broader agreement, including on the issue of "transparency," demanding a Chinese commitment to allow some kind of oversight to verify its actions to control emissions of carbon dioxide and other greenhouse gases.
The Chinese thus far have resisted what they see as a potential intrusion on their sovereignty. But without that, Clinton told reporters, "there will not be the kind of concerted global action that we so desperately need."
Clinton's arrival and announcement in snowy Copenhagen ratcheted up the U.S.-Chinese diplomatic dueling that has marked the two weeks of climate talks. The negotiations end Friday with a summit gathering of President Barack Obama, Chinese Premier Wen Jiabao and more than 110 other national leaders.
For China's part, a Foreign Ministry spokeswoman in Beijing told reporters Thursday that developed countries should show "more sincerity" in the talks here.
Environment ministers, having taken over from lower-level negotiators, were getting down to final hours of talks Thursday in hopes of producing partial agreements to put before Obama, Wen and the others leaders.
Such accords might include long-term goals for financing climate aid, raised by Clinton, and monitoring of emissions controls.
The Danish hosts had envisioned a comprehensive Copenhagen deal listing emissions cuts by richer nations, other restraints on the production of greenhouse gases by major developing countries, and a plan to help finance poorer countries adapt to global warming. It was to have served as a framework for a treaty to be completed next year.
But organizers of the U.N. conference probably will not get the climate deal they had hoped for, one Danish official said Thursday. The official was not authorized to speak publicly about the talks and spoke on condition of anonymity.
British Prime Minister Gordon Brown was among those stressing the time left, not the time lost.
"We can, by working together over the next 48 hours, reach agreement that will help the planet move forward for generations to come," he told reporters.
Two weeks of detailed talks on a range of issues - from emissions commitments, to preventing deforestation, to transferring clean-energy technology - reached an impasse on Wednesday when developing nations objected to the process that produced a core draft document.
In a reprise of a perennial complaint, the poorer nations said they were being excluded from the drafting of the text, that "northern" - read wealthy nations' - views were being imposed on the "south," or developing nations.
The Clinton offer on long-term climate financing for developing countries reflected an amount - $100 billion - that Britain's Brown has previously suggested, to help poorer countries build sea walls against rising oceans, cope with unusual drought and deal with other impacts of climate change, while also financing renewable-energy and similar projects.
"It's good there's now been a statement of support for a clear number on long-term finance," U.N. climate chief de Boer said of the U.S. offer. "This discussion will have to take place with other parties, whether they feel that sum is adequate."
Expert studies, by the World Bank and others, have estimated the long-term climate costs for poorer nations, from 2020 or so, would likely total hundreds of billions of dollars a year. China and other developing countries say the target should be in the range of $350 billion.
More immediately, the conference has been discussing a short-term climate fund to help developing countries - a $10-billion-a-year, three-year program. European Union leaders last week committed to supplying $3.6 billion a year through 2012. On Wednesday, Japan, seeking to "contribute to the success" of Copenhagen, announced it would kick in $5 billion a year for three years.
U.S. funding is hovering at only around $1 billion this year, and Clinton, when asked did not specify how much Washington would contribute to the "fast start" package.
"We'll do our proportion of `fast start'," the secretary of state said.
De Boer commented afterward, "I'm keenly looking forward to hearing what the U.S. contribution to that fund will be."
German Chancellor Angela Merkel, meanwhile, said the U.S. must improve its offer of emission cuts and stressed the urgency of reaching a concrete climate agreement in Copenhagen.
"I have to be honest, an offer by the United States to cut only 4 percent from 1990 levels is not ambitious enough," Merkel told lawmakers in Berlin before heading off to Copenhagen. "I believe this Copenhagen conference is the primary touchstone for whether we will succeed in setting a new path of global development, of sustainability."
The EU has pledged a 20 percent emissions cut that could increase to a 30 percent cut if other developed nations also make far-reaching pledges.
Associated Press writers Jan M. Olsen and John Heilprin contributed to this report.
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