COPENHAGEN (AP) - The United States and China searched for common ground to invigorate the U.N. climate talks in Copenhagen and deliver an agreement that President Barack Obama and Premier Wen Jiabao can sign at Friday's historic climate summit.

Fresh off a plane from Washington, Secretary of State Hillary Rodham Clinton announced that the U.S. would join others in raising $100 billion a year by 2020 to help poorer nations cope with global warming. That's a "good first step," China's vice foreign minister, He Yafei, said later.

Clinton made the offer contingent on reaching a broader agreement at the 193-nation conference that covers "transparency," a reference to U.S. insistence that China allow some international review of its actions controlling emissions of carbon dioxide and other greenhouse gases.

Going some way to meet U.S. demands, the Chinese official told reporters that Beijing was ready for "dialogue and cooperation" on its emissions actions "that is not intrusive, that does not infringe on China's sovereignty."

China's seemingly willingness to enhance the verifiability of its actions was "a clear shift in their position," said Srinivas Krishnaswamy of Greenpeace.

The diplomatic duel between Washington and Beijing has marked two-weeks U.N. climate talks in Copenhagen, which ground to a near-halt as a chronic rich-poor divide flared into the open again. That dimmed the hopes of the Danish hosts for a comprehensive deal - a preliminary framework for a formal treaty next year on combating climate change.

Environment ministers, having taken over from lower-level negotiators, got down to the final hours of talks Thursday in hopes of producing partial agreements to put before Obama, Wen and more than 110 other leaders at today's summit.