Clinton told reporters that verifying emission reductions is key to a successful agreement. She said the U.S. financial contributions, key for the support of poorer nations that fear they would suffer economically if forced to cut emissions, would be in jeopardy "in the absence of transparency from the second biggest emitter -- and now I guess the first, biggest emitter," a reference to China.
A senior administration official said today the "agreement is contingent on a strong overall agreement that includes transparency."
The White House said this morning that the elements of an agreement are in place, and hopes China will get on board but expressed concerns about China's commitment to transparency.
"The elements of getting an agreement are there if countries like China will make some common sense agreements about transparency," Gibbs said. "If the Chinese are unwilling to prove, to be able to prove to the world that they can live up to the agreement that they make, then I think it calls into question whether or not you truly have an agreement."
Obama has laid out a goal of reducing U.S. greenhouse gas emissions 83 percent by 2050, with a benchmark of a 30 percent reduction from 2005 emission levels by 2025.
But Congress is still working out the details on the goals for emissions reduction by 2015. In June, the House passed a bill that mandated a 17 percent cut by 2020, but so far the Senate has not considered the issue.
Clinton also met with leaders from island nations who fear rising ocean levels caused by global warming will sink their countries within the coming decades. Her other meetings today included officials from India, who rebuffed her efforts to secure an agreement on emissions last summer, Brazil, and South Africa.
Sen. John Kerry, D-Mass., chairman of the Senate Foreign Relations Committee, was in Copenhagen this week for the summit and said funding for global climate financing is a "critical component of any agreement" reached there.
"Financing is a major step forward and now we need to reach common ground on decisive national mitigation actions and transparency that provide credibility to the entire process," Kerry said in a statement. "China needs to rejoin the effort and start playing a constructive role."
ABC News' Yunji de Nies and Sunlen Miller contributed to this report.