The US has offered no new targets to cut greenhouse gas emissions, despite being the world's second biggest carbon emitter.
President Barack Obama has told the UN climate change summit in Copenhagen that a deal at the summit "hangs in the balance".
Some 193 countries and more than 120 world leaders are meeting in the Danish capital on the final day of the two-week conference.
He has also met the Chinese President at the summit.
The US and China are at loggerheads over their respective commitments to a successor to the Kyoto Treaty.