A senior Obama administration official said the announcement was carefully timed to resuscitate the talks before Mr. Obama’s arrival.

Image Secretary of State Hillary Rodham Clinton with Prime Minister Gordon Brown of Britain during the World Climate Conference in Copenhagen on Thursday. Credit... Pool photograph by Peter Macdiarmid

“It’s a negotiation,” he explained.

But Mrs. Clinton’s offer came with two significant conditions. First, the 192 nations involved in the talks here must reach a comprehensive political agreement that takes effect immediately. Second, and more critically, all nations must agree to some form of verification  she repeatedly used the term “transparency”  to ensure they are meeting their environmental promises.

China, the world’s largest producer of greenhouse gases, has brought the talks to a virtual standstill all week over this issue, which its leaders claim to be an affront to national sovereignty.

But the Chinese balkiness on the issue is matched in large measure by Mr. Obama’s own constraints. The Senate has not yet acted on a climate bill that the president needs to make good on his promises of emissions reductions and on the financial support that he has now promised the rest of the world.

“The president and his team have been doing everything possible to create a deal that is fair to the U.S. and facilitates international agreement,” said Paul Bledsoe of the National Commission on Energy Policy, a bipartisan advisory group. “But if the Chinese will not accept monitoring of emissions, then a deal is not worth doing.”

China appeared to crack the door a bit toward a system of reporting its emissions and its actions to reduce them on Thursday. He Yafei, the vice foreign minister, repeated China’s opposition to any intrusive international monitoring regime in a news conference on Thursday. But he said his country would consider voluntary “international exchanges” of information on its climate programs.

Administration officials here were not ready to publicly declare any breakthroughs in their talks with China and other nations on verification measures.