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 enable poor nations to 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 such an extensive financing effort, even though she did not specify the amount the United States would contribute along with other nations. 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.

But Mrs. Clinton warned that the United States would not participate in such a fund-raising effort without certain assurances from China.

“It would be hard to imagine, speaking for the United States, that there could be the level of financial commitment that I have just announced in the absence of transparency from the second-biggest emitter — and now I guess the first-biggest emitter — and now nearly, if not already, the second-biggest economy,” Mrs. Clinton said.