"We doubt that we can get to the legally binding agreement that everyone wants because too many countries have too many questions," she told Filipino university students in Manila.

The 7-18 December conference aims to draft a pact that would replace the 1997 Kyoto Protocol, which President George W. Bush’s prior administration opposed. It expires in 2012.

The US signed up to the protocol in 1998 under then President Bill Clinton, but never managed to ratify it. In 2001 Bush pulled the US out altogether, claiming it gave huge economic advantages to countries such as India and China, which did not sign up.

Clinton says the US game plan is to seek a strong “framework agreement” that could form the basis for an eventual legally-binding accord that the UN or other organizations would enforce.

"We are going to go to Copenhagen 100 percent committed to creating a framework agreement," she says.

US influence at the summit may be undermined by failure of Congress to pass legislation with even modest measures to slow global warming, which could limit its ability to commit to ambitious carbon dioxide reduction targets sought by Europe.

If Congress fails to act, the US Environmental Protection Agency intends to impose limits on carbon dioxide emissions under the 1970 Clean Air Act. That approach would likely face numerous legal challenges that could delay its full implementation.

Clinton urged European countries to tone down their ambitions for a full treaty at Copenhagen, arguing that the “pursuit of perfection” should not get in the way of progress.

"If we all exert maximum effort and embrace the right blend of pragmatism and principle, I believe we can secure a strong outcome at Copenhagen and that would be a stepping stone toward full legal agreement," she says.

Clinton emphasized that an eventual treaty “will not necessarily come quickly or easily."

The framework agreement sought by President Barack Obama’s negotiating team will need to include specific pledges, she says.

They include a commitment to lower greenhouse gas emissions; to assist developing nations with a global climate fund; to transfer necessary technology and to ensure accountability that domestic targets are being met.