United Nations talks on fighting climate change were hanging in the balance on Friday, US President Barack Obama told world leaders in Copenhagen, DPA reported.
"As the world watches us today, our ability to take collective action is in doubt and hangs in the balance," Obama told around 120 heads of state and government at crunch talks on climate change.
Obama has won acclaim for bringing the US back into world climate talks after years of deadlock under his predecessor, George W Bush.
"The question before us is no longer the nature of the challenge, the question is our capacity to meet it," he said.
But diplomats at the talks said that rows between the US and China, the world's most powerful developing nation, threatened to derail talks.
Obama said his country would continue to cut greenhouse-gas emissions "no matter what happens here in Copenhagen."