COPENHAGEN — U.S. Secretary of State Hillary Rodham Clinton sought to put new life into flagging U.N. talks today by announcing the U.S. would join others in raising $100 billion a year by 2020 to help poorer nations cope with global warming.

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 said, "there will not be the kind of concerted global action that we so desperately need."